HomeMy WebLinkAboutMS4 & TMDL Annual Report FY2425_FINAL
Credit: Nettle Creek, Lake Oswego, OR (M. Charbonneau)
November 28, 2025
2024-2025
NPDES MS4/TMDL Annual Report
City of Lake Oswego
National Pollutant Discharge Elimination System
Municipal Separate Storm Sewer System
Annual Report
Covering
July 1, 202 4 to June 30, 2025
Prepared for the Oregon Department of Environmental Quality November 30, 2025
CITY OF LAKE OSWEGO
NATIONAL POLLUTANT DISCHARGE ELIMINATION SYSTEM (NPDES)
MUNICIPAL SEPARATE STORM SEWER SYSTEM (MS4)ANNUAL REPORT
JULY 1,2024—JUNE 30,2025
I,the undersigned, hereby submit this National Pollutant Discharge Elimination System (NPDES)
Municipal Separate Storm system (MS4) annual report in accordance with NPDES Permit Number
101348. 1 certify, under penalty of law, that this document and all attachments were prepared under my
direction or supervision in accordance with a system designed to ensure that qualified personnel
properly gather and evaluate the information submitted. Based on my inquiry of the person, or persons,
who manage the system, or those persons directly responsible for gathering the information,the
information is,to the best of my knowledge and belief,true accurate, and complete. I am aware that
there are significant penalties for submitting false information including the possibility of fine and
imprisonment for known violations.
Mart a Bennett, City Manager Date
City of Lake Oswego
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TABLE OF CONTENTS
1 INTRODUCTION .................................................................................................................................... 1
2 TOPOGRAPHY AND HYDROLOGY .............................................................................................................. 1
3 ADAPTIVE MANAGEMENT ...................................................................................................................... 2
4 STORMWATER FUNDING ........................................................................................................................ 2
5 IMPERVIOUS AREA ............................................................................................................................... 3
5.1 Impervious Area Analysis (2019) ..................................................................................... 3
5.2 Legislative Changes ........................................................................................................ 4
5.3 Annexations ................................................................................................................... 5
5.4 Land-Use Review ............................................................................................................ 6
6 STORMWATER FACILITIES ....................................................................................................................... 7
6.1 Private Stormwater Facilities .......................................................................................... 8
6.1.1 Construction Plan Review ............................................................................................... 8
6.1.2 Construction ................................................................................................................... 8
6.2 Public Stormwater Facilities .......................................................................................... 10
6.2.1 Parks and Recreation Projects ...................................................................................... 10
6.2.2 Public Improvements .................................................................................................... 10
6.2.3 Capital Improvement Program...................................................................................... 10
7 STORMWATER MANAGEMENT PLAN ...................................................................................................... 13
7.1 Illicit Discharge Detection and Elimination .................................................................... 14
7.1.1 IDDE ............................................................................................................................. 14
7.1.2 Spill Response .............................................................................................................. 15
7.2 Industrial and Commercial Facilities ............................................................................. 16
7.2.1 Strategy Update ........................................................................................................... 16
7.2.2 Development Review ................................................................................................... 16
7.2.3 Construction ................................................................................................................. 16
7.2.4 Inspections and Notifications........................................................................................ 17
7.3 Construction Site Runoff Control .................................................................................. 17
7.3.1 Regulatory Changes ...................................................................................................... 18
7.3.2 ESC Professionals.......................................................................................................... 18
7.3.3 Plan Review .................................................................................................................. 18
7.3.4 Inspections and Enforcement ....................................................................................... 18
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7.4 Public Education and Outreach ..................................................................................... 19
7.4.1 City Outreach ............................................................................................................... 19
7.4.2 Stormwater Education Partnerships ............................................................................. 21
7.4.3 Regional Coalition of Rivers and Streams ...................................................................... 21
7.4.4 Clean Rivers Coalition ................................................................................................... 22
7.4.5 Clean Water Partners ................................................................................................... 23
7.5 Public Involvement ....................................................................................................... 23
7.5.1 Volunteer Events .......................................................................................................... 23
7.5.2 Public Comment ........................................................................................................... 24
7.6 Pollution Prevention for Municipal Operations ............................................................. 24
7.6.1 Conveyance Infrastructure Maintenance ...................................................................... 24
7.6.2 Street Sweeping ........................................................................................................... 25
7.6.3 Winter Maintenance .................................................................................................... 25
7.6.4 Wastewater Infrastructure ........................................................................................... 26
7.6.5 Pesticides ..................................................................................................................... 26
7.7 Stormwater Facility Maintenance ................................................................................. 28
7.7.1 Public Stormwater Facilities .......................................................................................... 28
7.7.2 Private Stormwater Facilities ........................................................................................ 29
8 ENVIRONMENTAL MONITORING PROGRAM ............................................................................................. 30
8.1 Monitoring Plan Changes .............................................................................................. 31
8.2 Wet Weather Monitoring ............................................................................................. 32
8.3 INSTREAM MONITORING .............................................................................................. 32
8.4 Continuous Monitoring Stations ................................................................................... 33
8.4.1 Lost Dog Creek ............................................................................................................. 33
8.4.2 Springbrook Creek ........................................................................................................ 33
8.5 Macroinvertebrate Assessment .................................................................................... 34
8.5.1 Results ......................................................................................................................... 34
8.5.2 Stressors ...................................................................................................................... 39
8.6 Total Maximum Daily Loads .......................................................................................... 39
8.6.1 Bacteria ........................................................................................................................ 39
8.6.2 Dissolved Oxygen ......................................................................................................... 42
8.6.3 Chlorophyll α and pH .................................................................................................... 43
8.6.4 Biocriteria..................................................................................................................... 45
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8.7 Water-Quality Limited Streams..................................................................................... 45
8.7.1 Algal Blooms and Aquatic Weeds .................................................................................. 46
8.7.2 Biocriteria..................................................................................................................... 47
8.7.3 Dissolved Oxygen ......................................................................................................... 47
8.7.4 Temperature ................................................................................................................ 48
8.7.5 Metals .......................................................................................................................... 48
9 REFERENCES ..................................................................................................................................... 51
APPENDIX A SWMP IMPLEMENTATION STATUS, FY2223 .................................................................................. A1
APPENDIX B TMDL IMPLEMENTATION PLAN ANNUAL REPORT ............................................................................. BI
Acronyms ......................................................................................................................................... Biii
B.1 INTRODUCTION ........................................................................................................................... B1
B.2. TMDL Implementation Plan Revisions ......................................................................................... B2
B.3 Program Management ................................................................................................................. B2
B.4 TMDLs & Stormwater Management ............................................................................................. B2
B.5 Wasteload allocations .................................................................................................................. B4
B.5.1 Bacteria ........................................................................................................................ B4
B.5.2 Dissolved Oxygen ......................................................................................................... B4
B.5.3 Chlorophyll α and pH .................................................................................................... B5
B.5.4 Biocriteria..................................................................................................................... B5
B.6 Temperature ............................................................................................................................... B6
B.6.1 Temperature TMDLs ............................................................................................................. B6
B.6.2 Temperature Mitigation ........................................................................................................ B6
B.7 Adaptive Management .............................................................................................................. B13
B.8 Other Activities .......................................................................................................................... B13
B.9 References................................................................................................................................. B17
LIST OF TABLES
Table 1. Historical and Forecasted Stormwater Budgets, FY1920 to FY2425 ............................................. 3
Table 2. Annexations in Lake Oswego, FY2223 ......................................................................................... 5
Table 3. History of Annexations in Lake Oswego, FY1617 to FY2425......................................................... 5
Table 4. Effect of Land-Use on Impervious Area ...................................................................................... 6
Table 5. Stormwater Facility Hierarchy .................................................................................................... 7
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Table 6. Impervious Area Created or Replaced, FY2425 ........................................................................... 9
Table 7. Impervious Area Treated by Private Stormwater Facilities, FY2425 ............................................. 9
Table 8. City-Initiated Projects Affecting Stormwater, FY2425................................................................ 11
Table 9. Treatment of Impervious Area for Commercial and Industrial Projects ..................................... 17
Table 10. Pesticides Used on City-Owned Property ................................................................................ 27
Table 11. Public Vegetated Stormwater Facilities ................................................................................... 30
Table 12. Macroinvertebrate Sampling Sites .......................................................................................... 35
Table 13. Physical and Biological Characteristics of Selected Streams in Lake Oswego, 2021 .................. 37
Table 14. Bacterial Wasteload Allocations for Streams in Lake Oswego .................................................. 39
Table 15. E. Coli Concentrations in Ball Creek and Carter Creek ............................................................. 40
Table 16. E Coli. Concentrations in Springbrook Creek ..............................................................................41
Table 17. E. Coli concentrations in Nettle Creek, Lost Dog Creek, and the Boones Ferry sampling site .... 41
Table 18. Dissolved Oxygen, TSS, and VSS concentrations in Ball Creek and Carter Creek ....................... 42
Table 19. Chlorophyll α and pH Wasteload Allocations for Streams in Lake Oswego .............................. 43
Table 20. Total Phosphorous Concentrations and pH at Ball Creek and Carter Creek .............................. 43
Table 21. Total Phosphorous Concentrations and pH in the Oswego Lake Watershed ............................ 44
Table 22. Designated 303(d) Streams and Tributaries, 2022 ................................................................... 43
Table 23. Copper Concentrations for Selected Streams in Lake Oswego ................................................. 49
Table 24. Zinc Concentrations of Selected Streams in Lake Oswego ....................................................... 50
Table A.1. SWMP Implementation Progress ........................................................................................... A2
Table B.1 Watersheds affected by TMDLs of which the City is a Designated Management Agency..........B1
Table B.2. TMDL Parameters and City Activities Related to the NPDES MS4 Permit.................................B3
Table B.3. Stream Temperatures at Selected Sites in Lake Oswego, FY2425 and FY1718-FY2122............B7
Table B.4. Effective Shade Comparison between Protected and Non-Protected Riparian Areas ........... B12
Table B.5. Metrics for TMDL Management of Stream Temperatures .................................................... B14
LIST OF FIGURES
Figure 1. Impervious Area by Watershed, as calculated from 2019 LiDAR data......................................... 4
Figure 2. Wet Weather and In-Stream Monitoring Sites in Lake Oswego ................................................ 31
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Figure B.1. Canopy growth between 2014 canopy (dark green) and 2019 (bright green) ...................... B10
Figure B.2. Effective Shade (Protected and Non-Protected Areas) by Watershed ................................. B11
Figure B.3. Effective Shade in Sensitive Lands by Watershed, 2014 and 2019 ....................................... B11
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ABBREVIATIONS
ACEC American Council of Engineering Companies
ACWA Association of Clean Water Agencies
APWA American Public Works Association
AWWA American Water Works Association
ALT Text Alternative Text (used for individuals with visual or auditory disabilities)
CCTV Closed Circuit TV (used for video inspections)
CIP Capital Improvement Program
CRC Clean Rivers Coalition
CWA Clean Water Act
DEQ Oregon Department of Environmental Quality
DMA Designated Management Agency
DO Dissolved Oxygen
DOC Dissolved Organic Carbon
DOGAMI Oregon Department of Geology and Mineral Industries
DSL Department of State Lands
E. Coli Escherichia coli
ESC Erosion and Sediment Controls
ESU Equivalent Service Unit
FY Fiscal Year starting July 1, e.g. FY2223 means July 1, 2022 to June 30, 2023
GI Green Infrastructure
GIS Geographic Information System
HEP Habitat Enhancement Program
I/I Inflow and Infiltration
IDDE Illicit Discharge Detection and Elimination
IPM Integrated Pest Management
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KPTV Portland Metro TV station
LEED Leadership in Energy and Environmental Design
LID Low Impact Development
LiDAR Laser Imaging, Detection, and Ranging
LOFD Lake Oswego Fire Department
LU Land Use Case
M-IBI Macroinvertebrate Index of Biological Integrity
MS4 Municipal Separate Storm Sewer System
NFPA National Fire Protection Association
NPDES National Pollutant Discharge and Elimination System
O&M Operations and Maintenance
OAR Oregon Administrative Rule
ODA Oregon Department of Agriculture
OERS Oregon Emergency Response System
PFA/PFOA Per- and polyfluoroalkyl chemicals and perfluorooctanoic acid chemicals
PLRE Pollutant Load Reduction Evaluation
PREDATOR Predictive Assessment Tool for Oregon
PSA Public Service Announcement
PW Public Works
QA/QC Quality Assurance/Quality Control
R-5/ R-7.5 Residential lot zoning: 5,000 sq ft and 7,500-sq ft
RCCRS Regional Coalition for Clean Rivers and Streams
SDC System Development Charge
SFR Single Family Residential
SSO Sanitary Sewer Overflow
SWMM Stormwater Management Manual
SWMP Stormwater Management Plan
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TCLP Toxic Characteristics Leaching Procedure
TMDL Total Maximum Daily Load
TSS Total Suspended Solids
TVF&R Tualatin Valley Fire and Rescue
UIC Underground Injection Control
UGB Urban Growth Boundary
USB Urban Services Boundary
USACE US Army Corp of Engineers
USEPA US Environmental Protection Agency
USGS US Geological Survey
VSS Volatile Suspended Solids
WLA Wasteload Allocation
WO Work Order
WTP Water Treatment Plant
WWTP Wastewater Treatment Plant
WY Water Year
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 1
1 INTRODUCTION
The Clean Water Act (CWA) was enacted in 1972 to “restore and maintain the chemical,
physical, and biological integrity of the Nation’s waters”. After point sources such as industrial
facilities and wastewater treatment plants were regulated as part of the National Pollutant
Discharge Elimination System (NPDES) program, the USEPA found that the nation’s rivers and
streams were still not providing sufficient water quality to meet their assigned beneficial uses.
As a consequence, the NPDES Municipal Separate Storm Sewer System (MS4) Phase I permit
program was implemented in 1990 to regulate stormwater discharges from municipalities,
counties, and other entities that met the MS4 permit thresholds of a population of 100,000
people or an urbanized area. While the City of Lake Oswego (hereafter “City”) had a 1995
population of 34,000 people, it was considered part of an urbanized area and subject to the
NPDES program.
The City received its first NPDES MS4 Phase I permit (hereafter “MS4 permit”) from the Oregon
Department of Environmental Quality (DEQ) in 1995 as a co-permittee of the Clackamas County
permit (#101348). It includes the jurisdictions of Clackamas County, Gladstone, Happy Valley,
Johnson City, Lake Oswego, Milwaukie, Oak Lodge Sanitary District, Oregon City, Rivergrove,
West Linn, and Wilsonville.
In October 2021, DEQ issued the current MS4 permit. The City’s Stormwater Management Plan
(SWMP), outlining how the City will comply with the updated permit requirements, received
DEQ approval in February 2023. The SWMP, along with the permit, annual reports, and other
stormwater related documents are available on the City’s website.
FY2425 Update: There was a personnel and responsible official change from Sonja Johnson to
Megan Charbonneau on January 13, 2025.
2 TOPOGRAPHY AND HYDROLOGY
The City is bounded by I-5 on the west, the Willamette River on the east, a north ridge, and a
south ridge. The highest elevations range from approximately 1,000 ft in the north to over 700
ft along the south ridge. Between these two ridges lies Oswego Lake which is kept at an
elevation of 98 ft by the Oswego Lake dam. The elevation of property next to the Willamette
River is less than 10 ft whereas property next to the Tualatin River (south of the southern ridge)
is approximately 100 ft. The west side of the City ranges from 150 ft in the southwest to 200 ft
in the northwest.
Several scarp lines and historic landslides are within the City’s urbanized area. According to a
DOGAMI study (Burns et.al., 2013), 7% of the City’s area is at high risk for shallow landslide
activity and 2% is at high risk for deep landslide activity. These geological characteristics have
implications for stormwater management within the City.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 2
The City’s jurisdictional boundary (11.5 sq. miles) encompasses portions of the Willamette River
and Tualatin River basins. For the purposes of this report, the Willamette River is further broken
into four minor watersheds: Tryon Creek (a sub-watershed discharging directly to the
Willamette River), Springbrook Creek (discharging to Oswego Lake), Oswego Lake, and the
remaining Willamette River watershed area. The Tualatin River watershed in the city
encompasses the Ball Creek and Carter Creek watershed in addition to the southern section of
the City which drains to the Tualatin River. Ball Creek and Carter Creek are separated from the
Tualatin River basin only in the water-quality section because the City has monitoring sites and
water-quality information in those watersheds.
The Lost Dog watershed, primarily referenced in the water-quality section of the report, is a
sub-watershed of Oswego Lake and is included in discussions of the Oswego Lake watershed
unless specifically indicated in the report.
3 ADAPTIVE MANAGEMENT
Adaptive management is allowed by the MS4 permit and is defined in the permit as a method
for refining and improving stormwater programs. The City made several minor adaptive
changes for FY2324 but did not seek any changes for FY2425.
4 STORMWATER FUNDING
The City’s stormwater program is funded primarily through monthly stormwater fees and is
based on the number of equivalent service units (ESU) assessed for a property. An ESU is the
average impervious area for residential properties and, in 1992, was determined to be 3,030 sq.
ft. A low density residential (SFR) property is assessed at 1 ESU and properties which are not
zoned as SFR are assessed using the amount of impervious area on their property. For example,
a non-SFR property with 6,060 sq. ft. of impervious area is assessed at the rate of 2 ESUs per
month. The monthly stormwater fee per ESU was $21.60 for FY2324 and was $23.11 for
FY2425.
Stormwater system development charges (SDCs) are used to alleviate capacity issues but, since
the City is largely developed, they comprise a small portion of the total stormwater budget.
SDCs were $195 for FY2324 and were $197 in FY2425 for SFR projects. Non-SFR projects are
based on the number of ESUs multiplied by the SFR rate.
Since FY1920, the City has spent an average of $10.3 million annually on stormwater operations
and management, capital projects, parks facilities, and natural resource management. In
FY2425, the City spent approximately $20.7 million dollars for improvements in its parks and
natural areas and $5.2 million dollars on capital projects and stormwater operations and
maintenance (see Table 1). An additional $143,000 was spent on private natural resources
management.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 3
Table 1 does not include the funds from related programs such as the Planning Department’s
Urban Forestry programs and code enforcement; and the Park and Recreation Department’s
Master Plans, and park maintenance (not related to natural resource management). These
programs positively affect water quality and are used for stormwater education as well as other
objectives.
Table 1. Historical and Forecasted Stormwater Budgets, FY1920 to FY2526
Budget Component
FY1920
Actual1
FY2021
Actual1
FY2122
Actual1
FY2223
Actual1
FY2425
Actual1
FY2526
Budgeted1
PW Program Operations $ 875,000 $ 918,000 $1,209,000 $ 1,033,000 $1,294,000 $1,130,000
PW Program Management $1,336,000 $1,224,000 $1,213,000 $ 1,183,000 $1,313,000 $1,625,000
PW Capital Projects $2,243,000 $ 603,000 $ 978,000 $ 628,000 $1,767,000 $5,238,000
Parks Capital Projects3 - $1,626,000 $2,316,000 $2,377,0002 $20,740,0003 --
Public Riparian Restoration $ 278,000 $ 189,000 $ 183,000 $ 226,000 $ 323,000 $ 330,000
Private Riparian Restoration
(Watershed Councils) $ 91,000 $ 127,000 $ 102,000 $ 89,000 $ 60,000 $ 60,000
Sensitive Land Reviews - - - $ 4,000 $ 33,000 $ 40,0004
Backyard Habitat Program $ 38,000 $ 42,000 $ 44,000 $ 47,000 $ 50,000 $ 54,000
Total $4,861,000 $4,729,000 $6,045,000 $ 5,583,000 $ 25,580,000
1 - These numbers are approximate only and not used for auditing purposes; 2 – Paid through Parks Bonds; 3 – Includes funds
spent on utilities and structures not providing stormwater management; 4 – This budget item is on-call/as -needed;
PW=Public Works
5 IMPERVIOUS AREA
Impervious area can be changed through legislation, annexations, and land-use cases in
addition to construction. There were no changes in the Urban Services Boundary (USB) in
FY2425 nor are any expected in FY2526.
5.1 IMPERVIOUS AREA ANALYSIS (2019)
In 2021, the City categorized its impervious area using the LiDAR data flown in 2019. The data
showed a total impervious area of 2,847 acres for 2019. The Oswego Lake watershed had the
greatest amount of impervious area and Lost Dog Creek had the smallest amount of impervious
area (See Figure 1). Buildings and plazas constitute 39% of the impervious area in the city with
transportation-related impervious area (bridges, railroads, and streets) a close second at 25%.
Graveled areas, such as alleys, constituted the least amount of impervious area at 0.7%.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 4
The City expects to complete another impervious area analysis using the LiDAR data from the
2024 flight. Preliminary data will be available in the spring of 2025 however the City does not
expect to have full use of the data until 2026.
Figure 1. Impervious Area by Watershed, as calculated from 2019 LiDAR data
5.2 LEGISLATIVE CHANGES
The municipal code was reviewed for compliance with the 2021 MS4 permit in FY2425. It
resulted in four categories of changes: 1) updating language, e.g. stormwater runoff was
changed to either stormwater or runoff, 2) merging definitions when applicable and either
adding or deleting as necessary, 3) consolidating requirements, e.g. referring to the primary
code chapter instead of restating the requirements in several places, and 4) changing
requirements specific to Chapter 52 (Erosion Control) and Chapter 38 (Utility Code).
In Chapter 52 (Erosion Control) and Article 38.25 (Stormwater Management), code
requirements were removed when they were 1) process-oriented (such as the steps for a
stormwater review), 2) the enforcement responsibility of other jurisdictions (such as DEQ's
1200-C permit), or 3) out-of-date such as sending private stormwater maintenance records to
staff. A variance procedure with approval criteria was added to the code as well as changing the
enforcement procedures to align with each other and to ensure an escalating enforcement
procedure.
0
100
200
300
400
500
600
700
800
900
Ball Creek Carter Creek Lost Dog
Creek
Oswego
Lake
Springbrook
Creek
Tryon Creek Tualatin
River
Willamette
River
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 5
Workshops were held for the Planning Commission and the City Council in June 2024. The
Planning Commission recommended the proposed code revisions in September 2024 to the City
Council. The City Council adopted the proposed code amendments in November 2024 with an
effective date in December 2024.
5.3 ANNEXATIONS
The City has approximately 13 sq. miles within its urban service area and 11.5 sq. miles within
its jurisdiction. Four properties (4 lots) were annexed into the City in FY2425 (see Table 2) with
future development as the primary reason being cited for annexation.
Table 2. Annexations in Lake Oswego, FY2425
Potential Lots
Property
Area
(acres) Watershed
Pre-
annexation
Post-
Annexation Reason Effect
13699 Knaus 0.54 Tryon Creek 1 1 Future Development
Increased
impervious area
13092 Amber
Place 1.23
Willamette
River
Watershed 1 1
Connection to public
wastewater and
future development
Increased
impervious area
16523 Bonaire
Avenue 0.23
Oswego
Lake 1 1
Connection to public
wastewater
Connection to
public
wastewater
5077 Dawn 0.24 Tryon Creek 1 1
Connection to public
wastewater and
future development
Increased
impervious area
Total 2.24 4 4
From FY1617 to FY2425, an average of 8 annexations encompassing an average of 6.3 acres
annually (see Table 3).
Table 3. History of Annexations in Lake Oswego, FY1617 to FY2425
Year
Number of
Annexations
Acres
Annexed Year
Number of
Annexations
Acres
Annexed
FY25261 6 3.49 FY2021 12 10.8
FY2425 4 2.24 FY1920 8 8.2
FY2324 6 7.8 FY1819 13 6.6
FY2223 5 2.9 FY1718 9 4.0
FY2122 8 8.4 FY1617 11 6.6
1 – Approved annexations as of 10/29/25.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 6
5.4 LAND-USE REVIEW
In FY2425, there were 18 applications for variances, partitions, and lot line adjustments
affecting 14 acres (see Table 4). The approved applications resulted in an additional 6 lots
created in the city. Increased impervious area will result in all but seven of the approvals. A
majority of the cases were in the Oswego Lake watershed and affected 8.2 acres. Cases in Tryon
Creek affected 3.9 acres, 1.59 acres in Springbrook, and .23 acres in Willamette River
watershed.
Table 4. Effect of Land-Use on Impervious Area
LU Case Watershed Acreage
Potential Number
of Lots
Change
Impervious
Area Effect Previous Current
LU 24-0015 Oswego Lake 0.48 1 1 Variances Increase
LU 23-0042 Springbrook 1.30 3 4 4-Lot subdivision Increase
LU 24-0010 Tryon Creek 0.73 1 2 2-Parcel Partition Increase
LU 24-0028 Oswego Lake 0.14 1 1 Variances Increase
LU 24-0021 Tryon Creek 0.98 1 2 2-Parcel Partition Increase
LU 24-0040 Oswego Lake 0.19 1 1 Variances No Effect
LU 24-0047 Oswego Lake 0.32 1 1 Variances No Effect
LU 24-0051 Oswego Lake 0.37 1 1 Variances Increase
LU 25-0008 Oswego Lake 1.14 1 1 Lot Line Adjustment No Effect
LU 23-0046 Tryon Creek 2.20 1 3 3-Parcel Partition Increase
LU 25-0005 Oswego Lake 0.38 1 1 Variances Increase
LU 24-0049 Willamette River 0.23 1 1 Variances Increase
LU 25-0009 Oswego Lake 0.66 1 1 Lot Line Adjustment No Effect
LU 25-0012 Springbrook
Creek 0.29 1 1 Lot Line Adjustment No Effect
LU 25-0011 Oswego Lake 0.30 1 1 Variances No Effect
LU 24-0038 Oswego Lake 0.18 1 1 Variances No Effect
LU 24-0046 Oswego Lake 3.80 1 2 2-Parcel Partition Increase
LU 24-0033 Oswego Lake 0.28 1 1 Variances Increase
Total 13.97 20 26
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 7
Not all of the variances and lot line adjustment cases resulted in a requirement for stormwater
management. Additionally, not all land-use cases result in a partition, variance, or lot line
adjustment. In FY2425, stormwater management was required for 37 development
applications. Most were for SFR projects.
6 STORMWATER FACILITIES
Stormwater facility design is governed by the Stormwater Management Manual (SWMM) and
the Engineering Design Standards. Projects are submitted for compliance with these standards
prior to construction.
While there is some overlap due to update schedules, the Engineering Design Standards
primarily cover stormwater conveyance structures whereas the SWMM is the primary driver for
stormwater management and treatment. There were no changes in FY2425 to the Engineering
Design Standards and Detail Drawings. The SWMM and the Engineering Design Standards and
Detail Drawings are currently being updated and are expected to be implemented in 2026.
A review and update of the LID/GI strategy was required as part of the 2021 MS4 permit and
was submitted on December 1, 2023. In summary, the strategy requires infiltration of the 10-yr
24-hr storm event using infiltration facilities as a first tier of stormwater management (see
Table 5). The second tier of management requires extended filtration of stormwater prior to
discharging offsite but requires a technical reason for not using Tier 1 facilities. The third tier
allows the use of all facilities approved in the City’s SWMM but requires technical reasons for
not using Tier 1 and Tier 2 facilities.
Table 5. Stormwater Facility Hierarchy
Tier Approved Stormwater Facilities
Tier 1
(Onsite Retention)
Infiltration Planters
Infiltration Raingardens
Pervious Pavers
Pervious Pavement
Infiltration Trenches or Galleries
Drywells
Tier 2
(Extended Filtration)1
Filtration Planters
Filtration Raingardens
Swales
Water-Quality Vaults (non-SFR projects only)
Tier 3
All other technologies approved for use and meeting the
requirements of the SWMM
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 8
6.1 PRIVATE STORMWATER FACILITIES
The SWMM requires onsite stormwater management for the 10-yr 24-hr design storm
whenever technically feasible. Projects that create or replace 3,000 sq ft or more of impervious
surface area are expected to meet both flow and water-quality requirements, except for
projects that (1) discharge directly, or through manmade infrastructure, to the Tualatin River,
Oswego Lake, or the Willamette River and (2) are located in areas with no downstream capacity
issues.
6.1.1 Construction Plan Review
At the construction plan review stage, a draft operations and maintenance (O&M) plan must be
submitted and, if approved, recorded with the County. Projects using infiltration trenches,
infiltration galleries, and drywells are required to show that they have started the process for
DEQ approval of their underground injection controls (UIC) when they submit construction
plans for staff review. Applicants must submit copies of the recorded O&M Plan and the DEQ
approval to Engineering prior to approval of the final erosion and sediment control inspection.
In FY2425, staff reviewed 53 construction plans of which 44 were classified as large projects
(3,000 sq. ft. or more of new and/or replaced impervious area).
6.1.2 Construction
After a construction plan is approved, the private stormwater design engineer must inspect the
construction of the facilities and provide a signed certification to the City at project completion
stating that construction of the stormwater facility adhered to the design. Before the end of
construction, a copy of DEQ’s approval for all constructed UICs must be submitted to the City.
At the end of construction, a copy of the recorded O&M plan must be provided to the City.
6.1.2.1 Stormwater Treatment by Watershed
Stormwater facilities were completed for four commercial projects (1.41 acres), 1 public
improvement project (1 acre), and 48 SFR projects (4.48 acres) resulting in 6.90 acres of created
or replaced impervious area (see Table 6). Most of the development (SFR and non-SFR) was in
the Oswego Lake watershed at 3.44 acres.
6.1.2.2 Stormwater Treatment by Facility Type
The City’s SWMM emphasizes infiltration of the 10-yr 24-hr design storm event (3.2 inches).
Because infiltration creates shallow groundwater (interflow) that replenishes streams during
the dry summers, it is expected that continued emphasize on infiltration facilities and onsite
stormwater treatment will assist the City with attaining the Willamette River temperature
TMDL that affects every watershed in the City’s jurisdiction.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 9
Table 6. Impervious Area Created or Replaced, FY24251.
Watershed
Number of
Projects
Impervious Area,
acres Non-SFR2, Acres SFR3, Acres
Oswego Lake 21 3.44 1.63 1.81
Springbrook Creek 6 0.78 0.10 0.67
Tryon Creek 13 1.55 0.57 0.98
Tualatin River 4 0.41 - 0.41
Willamette River 9 0.72 0.11 0.61
Total 53 6.90 2.41 4.48
1 – Does not include projects with < 1,000 sq ft of new impervious area or flatwork that does not require a building permit (pool
decks, sport courts, large patios; 2 – SFR means single-family residential but does not include middle housing projects; 3 – non-
SFR means all other projects including middle housing projects.
Infiltration galleries and drywells accounted for the largest portion of treated impervious area,
representing a combined 56.6% of the total (see Table 7). Filtration planters and a water quality
vault treated an additional 28.8% of the area.
During FY2425, 41.8% of stormwater treatment was achieved through green infrastructure (GI),
with infiltration facilities handling 67.6%. Underground Injection Controls (UICs) were used to
treat approximately 78.2% of the total impervious area.
Table 7. Impervious Area Treated by Private Stormwater Facilities, FY2425
Facility Type
Oswego
Lake
Springbrook
Creek
Tryon
Creek
Tualatin
River
Willamette
River
Total by Facility
Type
Acres Acres Acres Acres Acres Acres
% of
Total
Raingarden, Infiltration 0.51 - - - - 0.51 7.4
Raingarden, Filtration - - 0.08 - - 0.08 1.2
Planter, Infiltration - - 0.12 - 0.11 0.23 3.3
Planter, Filtration 0.33 0.10 0.25 - 0.41 1.09 15.8
Pervious Concrete - - - - - - -
EcoRoof - - - - - - -
Bioswale - - 0.18 - - 0.18 2.6
Drywell 0.62 - 0.50 0.15 - 1.27 18.4
Infiltration Trench - - - - - - -
Infiltration Gallery 1.08 0.68 0.42 0.26 0.20 2.64 38.2
Water-Quality Vaults 0.90 - - - - 0.90 13.0
Sheet Flow - - - - - - -
Pond - - - - - - -
Untreated - - - - - - -
Total by Watershed 3.44 0.78 1.55 0.41 0.72 6.90
Green Infrastructure1 0.84 .10 0.63 - 0.52 41.8
Infiltration2 1.59 0.68 0.54 0.26 0.31 67.6
UICs3 1.70 0.68 0.92 0.41 0.20 78.2
1 –Includes planters, raingardens, porous pavers, pervious pavement, bioswales, and EcoRoofs ; 2 – Includes infiltration
raingardens, infiltration planters, permeable pavers, infiltration trenches, and infiltration galleries; 3 – Includes drywells,
infiltration galleries, and infiltration trenches.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 10
6.2 PUBLIC STORMWATER FACILITIES
Public stormwater facilities are built either through public improvements required by land-use
decisions, the Public Works Capital Improvement Program (CIP), or the Parks and Recreation
(Parks) capital improvements. The CIP constructs facilities in areas without stormwater
system(s) or to increase capacity of existing infrastructure. Parks is required to build
stormwater facilities to manage runoff from impervious area in their parks, natural areas, and
facilities, e.g. parking lots or sports fields.
The City requires a video inspection of any new stormwater system prior to the stormwater
facility to ensure that it meets requirements for structural integrity. In FY2425, the City required
video of approximately 4,400 feet of stormline.
6.2.1 Parks and Recreation Projects
During FY2425, three active Parks and Recreation projects were completed. The Lake Oswego
Aquatic Center and golf course include a flow-through rain garden and pond located within the
Lost Dog (Oswego Lake) watershed. Phase 1 of the Rassekh Park project was also completed,
featuring an infiltration rain garden in the Tualatin River watershed.
6.2.2 Public Improvements
Proposed public developments must meet SWMM requirements and some projects are
required to construct public stormwater facilities. Four projects were completed in FY2425 to
provide public improvements. These included the construction of a swale, a drywell, and two
infiltration planters. The drywell is located in the Tualatin watershed, the swale in the
Willamette watershed, one planter in the Tryon Creek watershed, and the other in the Oswego
Lake watershed. All projects were completed and are in the City's 1-yr warranty period.
6.2.3 Capital Improvement Program
Projects in the Capital Improvement Program (CIP) are often comprehensive in scope. They
often will involve the repair, replacement, or upgrade of multiple infrastructure system
including, wastewater, drinking water, stormwater, and pavement within a single project. This
integrated approach allows the City to optimize shared costs while minimizing transportation
disruptions to the public. Project timelines may be adjusted either delayed or accelerated
depending on available funding and right-of-way acquisition, infrastructure conditions, rate of
deterioration, and the ability to provide multi-utility improvements.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 11
Table 8. City-Initiated Projects Affecting Stormwater, FY2425
Status Project Watershed Description
Reduce
Sediment
Decrease
Hydromodification
Increase
WQ
Capital Improvement Projects
Design 5th St/Lake Bay (WO 346) Oswego Lake Decommission UIC. Treat stormwater. X X X
Design Redfern Ave (WO 355) Oswego Lake Decommission UIC. Treat stormwater X X X
Design UIC Retrofit #3 (WO 354) Tualatin1 Decommission UIC. Treat stormwater X X
Design
ADA Curb Ramps
(WO 347) Various Replace catch basins X X
Design
Glenwood Ct
(WO 369) Willamette Drainage improvements x x
Design
Pilkington Pathway
(WO 337 / WO 361) Oswego Lake Adding new stormline and catchbasins x x x
Design
Carman Pathway
(WO 337 / WO 363)
Tualatin and
Oswego Lake Install flow-through planter X X X
Construction
Tree Top / Meadowlark
(WO 337 / WO 362) Oswego Lake Add catch basin X X
Construction Lakeview Blvd (WO 333) Oswego Lake Drainage Improvements X X
Construction Atwater Outfall (WO 352) Tryon Creek Repair of failing outfall X
Construction 2025 Pavement Rehab (WO 358) Various Replace catch basins X X
Completed 2nd and E Ave (WO 350) Tryon Creek Add stormwater treatment X X X
Completed Tryon Creek Outfall (WO 351) Tryon Creek Abandon outfall and repair erosion X
Completed Jean / Pilkington (WO 273) Oswego Lake
Install stormwater planters, WQ vaults,
and sumped / snouted CBs. X X X
Completed
Daniel Way
(WO 323) Springbrook
Riparian restoration, velocity reduction,
and add sumped / snouted CBs. X X X
Completed
Lanewood Pathway
(WO 320 / WO 360) Springbrook
Decommission UIC. Add 4 sumped /
snouted CBs. X X
Completed
2024 Pavement Rehab
(WO 348) Various
Replaced 10 CBs with sump / snouted
CBs. X
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 12
Status Project Watershed Description
Reduce
Sediment
Decrease
Hydromodification
Increase
WQ
Parks and Recreation Projects
Completed
Lake Oswego Aquatic Center &
Golf Course Oswego Lake Flow-through raingarden and pond. X X
Completed Rassekh Park (phase 1) Tualatin Infiltration raingarden. X X X
1 – A small portion is in the Oswego Lake watershed; I/I – Infiltration and inflow; WO = Work Order; CB = catch basin; WQ = water-quality
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 13
Eight CIP projects were completed in FY2425 that had a stormwater component (see Table 8).
Two projects were located in the Oswego Lake, Tryon and Springbrook watershed, 1 in Tualatin
and the pavement rehabilitation projects were distributed across multiple watersheds.
6.2.3.1 Retrofit Projects
The 2015 Retrofit Assessment identified several retrofit criteria including protection of private
and public infrastructure, repair or maintenance of existing infrastructure, improvement of
water quality or natural resources, regulatory requirements, or the ability to be combined with
another CIP project (multi-utility). In 2023, the City updated its retrofit and its
hydromodification project list to comply with the 2021 MS4 permit.
The City has completed the Lakeview Blvd project listed in the 2015 Retrofit Assessment. It
included 3-ft sumped/snouted catchbasins and media filters for treatment. The Redfern project
was also listed in the 2015 Retrofit Assessment and will include stormwater treatment in 2026.
6.2.3.2 Hydromodification Projects
Hydromodification occurs as the volume and velocity of stormwater changes a waterway over
time. Methods for mitigating hydromodification are to increase infiltration or reduce
stormwater velocities. To a lesser extent, detention can address hydromodification because it
reduces the peak velocity of the stormwater however it can increase hydromodification if
stream velocities are at peak flows for an extended period of time. The extended peak flows
result in cohesion fatigue of soil particles in the streambed and streambank which leads to
erosion.
Five completed projects reduced hydromodification in FY2425. In addition, six engineering
projects currently under construction or in design are expected to further reduce
hydromodification. These projects are located in the Oswego Lake watershed (two projects),
the Springbrook Creek watershed, and the Tualatin River watershed (see Table 8).
7 STORMWATER MANAGEMENT PLAN
The Stormwater Management Plan (SWMP) provides the framework for the City’s
implementation of the MS4 permit and contains the minimum measures required in the MS4
permit along with measurable goals and tracking measures to assess progress in meeting
milestones and the requirements of the permit. The SWMP was approved by DEQ in February
2023.
Additional goals and tracking measures not covered in this section are available in Appendix A.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 14
7.1 ILLICIT DISCHARGE DETECTION AND ELIMINATION
The Illicit Discharge Detection and Elimination (IDDE) program includes an IDDE plan, spill
response, and training.
The City conducts Drug TakeBack events every year which decrease the amount of unwanted
and expired prescription drugs discarded in the trash or into the wastewater system. The
events are important because wastewater treatment plants (WWTPs) are not designed to
remove dissolved drugs and they can become a source of mutations or mortality in aquatic
species. In FY2425, staff held events that resulted in 407 pounds of drugs being removed from
the solid waste stream. On average, the City has removed an average of 811 pounds of drugs
from the solid waste stream each year since FY2021.
7.1.1 IDDE
The IDDE program includes outfall monitoring, cross-connection investigations, decommission
of septic systems, and spill response. Training occurs on an 2-year cycle regarding spill response
and hazardous material identification. In FY2425, 45 LOFD staff were trained on hazardous
material identification and 45 LOFD staff were trained on spill response procedures.
7.1.1.1 Outfall Monitoring
With the implementation of the new SWMP, the City is required to inspect 17 priority outfalls
on a 2-yr rotating basis. Staff were able to inspect all 17 priority outfalls during FY2425. All
outfalls were determined to be in “good”
condition. Outfalls that had active flow or
groundwater discharge were tested with a
handheld meter and were determined to not
have any indicator parameters to indicate any
issues with water quality.
There were no changes to the prioritization
criteria in FY2425.
7.1.1.2 Cross-Connections and Septic
System Decommissioning
Staff use public complaints and video to locate
cross-connections. There were no cross-
connection (wastewater to stormwater)
discovered in FY2425.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 15
Staff inspect new wastewater connections and monitor when septic systems are
decommissioned. Two septic systems were decommissioned in FY2425. One was located in the
Tualatin and the other in the Oswego Lake watershed.
7.1.2 Spill Response
PW and LOFD staff provide spill response, maintain the inventory of spill response materials,
and determine policies and procedures related to spill response. There were no changes in the
City’s Spill Response procedures, however, the contact list was updated. There were no changes
made to the City’s municipal code.
There were three reported spills in FY2425. One of the spills was
located in the Springbrook Creek watershed, one in the Tryon
watershed, and one was in the Tualatin River basin.
No citations were issued in FY2425 related to spill incidents. A
minor oil spill occurred in the Springbrook watershed; the spill
was contained and fully cleaned. In addition, a reported drain
spill at an apartment complex in the Tualatin watershed was
investigated after a delayed report from a resident. No evidence
of a spill was found, and educational materials were provided to
the residents. Lake Oswego High School, located in the Tryon
watershed was reported to DEQ for dumping heated and
treated swimming pool water into a catch basin on their property. DEQ contacted staff, they
have since received guidance on the proper disposal of swimming pool water as well as a
warning to the superintendent and all maintenance staff, maintenance staff fulfilled additional
educational requirements made by staff.
A car wash facility in the Oswego Lake watershed received warning letters for ongoing illicit
discharge. The car wash discharge has since been resolved.
The Lake Oswego Fire Department (LOFD) coordinated with Public Works Maintenance and
Operations to clean fluids released from automobile crashes. All fluids were properly contained
and cleaned within 24 hours of each incident.
The City had one SSO in FY2425 in the Springbrook Creek watershed. While most SSOs tend to
occur because of tree roots or private lateral failure, this incident was caused by the top of a
tree impaling the main line. Contractor was able to make timely repairs.
The Tualatin Valley Fire & Rescue (TVF&R) and LOFD have an agreement for consultation and
incident response for hazardous material spills. There were no incidents requiring consultation
or response by TVF&R in FY2425.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 16
7.2 INDUSTRIAL AND COMMERCIAL FACILITIES
The industrial and commercial facility program includes monitoring development for inclusion
in the DEQ Industrial Stormwater (1200-Z) program, conducting inspections of properties that
are not part of the 1200-Z program but which are in commercial or industrial zones, and
training. The City currently has two staff who have experience with business inspections but
none who hold current certifications.
7.2.1 Strategy Update
The City formalized the inspection and screening processes of its commercial and industrial
inspection program in 2023. Municipal code changes were necessary and staff provided
information about the program and the proposed municipal code changes to the City Council in
November 2023.
All non-SFR properties were sent a letter by mail requesting information via a survey and the
request is also posted on the City’s website to help prioritize industrial facility inspections.
7.2.2 Development Review
Sixteen (16) non-SFR projects underwent the land-use process in FY2425. Two were for school
and seven were for multi-family use. Of the remaining seven land-use cases, were commercial
buildings. It is not expected that any of the land-use cases will result in additional pollutant
loadings or be subject to DEQ’s 1200-Z program.
Two construction plans were reviewed in FY2425 for properties zoned as commercial and no
industrial projects. They were located in the Tualatin and Oswego Lake watersheds.
7.2.3 Construction
Three commercial and one institutional project was completed in FY2425 and treat a total of
9.85 acres of impervious area (see Table 9). No industrial projects were completed.
Three commercial projects affecting 5.18 acres were completed in FY2425. The commercial
projects were located in the Tualatin and Tryon Creek watersheds. To treat the 5.18 acres of
impervious, the stormwater facilities used were filtration planters, detention and drywell.
One Institutional project was completed in FY2425 affecting 4.67 acres. The project was located
in the Oswego Lake watershed. To treat the 4.67 acres of impervious, the stormwater facility
used was a flow through rain garden.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 17
Table 9. Treatment of Impervious Area for Commercial and Industrial Projects, FY2425
Facility
Oswego
Lake,
acres
Springbrook
Creek,
acres
Tryon
Creek,
acres
Tualatin
River,
acres
Willamette
River,
acres
Total,
Acres
Filtration Raingarden 4.67 - - - - 4.67
Filtration Planter - - 0.42 2.76 - 3.18
Infiltration Planter - - - - - -
Infiltration
Gallery/Trench - - - 1.83 - 1.83
Water-Quality Vault - - - 0.17 - 0.17
Total 4.67 - 0.42 4.76 - 9.85
7.2.4 Inspections and Notifications
The City conducted four 1200-Z screenings and sent letters to the DEQ and the facilities that
were suspected to require a 1200-Z permit. No updates were received from DEQ regarding the
facilities that were contacted.
During FY2425, staff completed nine
commercial inspections and worked closely
with commercial property owners to
educate them and ensure stormwater
systems were properly functioning or
repaired as needed. Most management
activities involved catch basin cleaning and
verifying that each commercial facility
maintained a current operations and
maintenance plan. The second most
common issue identified involved
bioswales, which often required regrading
to their original design and replanting to full vegetation coverage.
7.3 CONSTRUCTION SITE RUNOFF CONTROL
Construction site runoff control, or erosion and sediment control (ESC), is an important
program for reducing total suspended solids in streams and other waterways. The City currently
has a 500-sq. ft. soil disturbance threshold for ESC permits and also requires a permit for
projects that are within 50 ft. of a lake or waterway. The program consists of reviewing plans,
inspecting projects, and training.
There were four City-initiated projects (Parks and Engineering) with certified ESC professionals
as part of the project team in FY2425. The Lake Oswego Recreation and Aquatics Center was
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 18
completed in FY2425. Rassekh Park, Lake Oswego Middle School, and Lakeview Blvd. were
under construction in FY2425.
7.3.1 Regulatory Changes
Changes to the municipal code were adopted by the City Council in November 2024. Proposed
changes affect the enforcement actions, remove an exemption for residential landscaping, and
clarify what is required for an ESC Plan.
The ESC manual has been updated to add straw wattles as a permanent standard detail due to
common use in tree protection zones to avoid root damage per arborist input.
7.3.2 ESC Professionals
There are two staff who are certified ESC professionals. In FY2223, the City started requiring a
certified ESC professional to submit ESC plans for complex projects, e.g. non-SFR projects, to
reduce suspended solids. In FY2425, three ESC plans were submitted by certified ESC
professionals. Two of the projects were located in the Oswego Lake watershed and one was
located in the Tualatin River watershed.
7.3.3 Plan Review
The City issued 165 ESC permits in FY2425 with a majority of the permits classified as single-
family residential projects.
To assist with mercury and suspended solids reductions, the City requires coir matting in
riparian projects and for projects with slopes greater than 15%. In FY2425, three projects were
required to protect riparian areas using coir matting and four were required to provide
additional protection because of steep slopes. The riparian projects were located in Nettle and
Spirit Creek. All four projects with >15% slopes were located in the Tryon Creek Watershed.
The City requires projects affecting riparian areas to contact DSL/USACE to determine if the
project is subject to their permit. Applicants are required to provide a copy of the jurisdictional
decision to the City with the construction plans. Eight projects were required to contact
DSL/USACE before receiving their ESC permit; two were in the Oswego Lake watershed, three in
Willamette watershed, and three in Nettle Creek.
7.3.4 Inspections and Enforcement
A total of 192 inspections were completed after the installation of ESCs but prior to the start of
site construction. Final inspections, conducted once disturbed areas were stabilized, were
completed for 178 projects. Beginning in January 2023, the City increased inspection frequency
for projects active for six months or longer, resulting in 13 additional inspections during FY2425.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 19
One citation was issued for a project located in the Nettle Creek watershed during FY2425. In
addition, four stop-work orders were issued, two located in Nettle Creek and two in Lost Dog.
7.4 PUBLIC EDUCATION AND OUTREACH
The City participates in several regional stormwater outreach groups, including the Regional
Coalition of Clean Rivers and Streams (RCCRS), the state-wide Clean Rivers Coalition (CRC), and
Clean Water Partners. In addition, City staff promote stormwater education and outreach
through the City website, Farmers Markets, Stormwater Bingo, social media (Facebook,
Nextdoor, and Instagram), and City newsletters such as HelloLO (print) and LO Down (e-
newsletter).
7.4.1 City Outreach
In 2025, the City made a significant effort to increase public education and engagement related
to the stormwater program. The stormwater pages for the City were all updated throughout
2025, including an all new Education and Resources Page. The city also developed a social
media template and cohesive branding for its stormwater program to create a recognizable
identity and boost engagement and awareness.
The City reintroduced the Canines for Clean Water Pledge, encouraging responsible pet waste
cleanup. The program was reintroduced with updated content and promoted through social
media, with approximately 100 pledges handed out along with bandannas and tags.
A new Stormwater Bingo outreach campaign was designed to encourage people all ages to
explore local stormwater facilities, learn more about water quality, and participate for a chance
to win a plush fish prize each month. The campaign was launched in June 2025 and ran through
the end of summer 2025. Hundreds of Bingo cards were distributed and made available online,
at City Hall, the Library, the new Lake Oswego Aquatic and Recreation Center, Farmers Market
and Library children’s event.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 20
In 2025, the City introduced stormwater stickers
and a bilingual stormwater coloring book as
engaging, hands-on tools to promote stormwater
awareness and foster community pride in our city.
In 2025 City staff also attended one Lake Oswego
Farmers Market and one Lake Oswego Library event
focused on stormwater education, sharing practical
tips on car washing, local animals and critters found
in nearby streams, pet waste cleanup, and everyday
actions that impact local watersheds.
Members and followers increased significantly in
FY2425 on all platforms except for Twitter (X). The
City has over 8.2K followers on Facebook and 6.1K
followers on Twitter (X), respectively. Instagram
posts by the City reach over 12.5k members and
Nextdoor posts reach 26K members.
Social media activity has also increased, averaging two posts per month in 2025, compared to
one post per season previously. Topics have included seasonal stormwater management, such
as raking leaves and clearing tree debris from storm basins, eco-friendly de-icing, the impact of
native plants on stormwater, educational posts highlighting dog waste bags found during water
sampling, and information on volunteer programs and Canine for Clean Water pledge.
In FY2425, the City featured 27 articles, increasing stormwater education across the HelloLO
and LO Down newsletters, covering both new and recurring topics. New topics included pool
drainage, stormwater bingo, and native gardening. The newsletters also revisited important
seasonal and maintenance subjects such as pesticide and herbicide use, pet waste disposal,
boat dock upkeep, leaf removal from catch basins, drop-off boxes for leaf and tree debris, and
sediment cleaning in catch basins.
The City offers free water quality incentives such as hose timers, rain sensors, soil test kits, soil
moisture probes, and spray bottles for natural weed control as well as brochures providing
information on natural gardening. All of the items and brochures are accessible to the public by
requesting them through the telephone, email, or at the reception area.
Stormwater Bingo Winner! (2025)
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 21
Manuals, permits, annual reports, and other
stormwater-related documents are available
to the public through the City’s website or by
requesting them through telephone or email.
There were no comments on the documents
located on the City’s website in FY2425.
7.4.2 Stormwater Education
Partnerships
The City partners with several regional
stormwater education groups to provide
consistent messaging. The Regional Coalition
for Clean Rivers and Streams (RCCRS) has
provided stormwater messaging on a variety of topics to residents of the tri-county Portland
metropolitan area (estimated population of 1.8 million) for over 25 years. The Clean Rivers
Coalition (CRC) has over 60 partners across Oregon and Washington with a mission is to build a
bridge between clean water and healthy communities and provide consistent messaging across
Oregon and southwest Washington. The Clean Water Partners work with a local TV station,
KPTV, to provide public service announcements (PSAs).
7.4.3 Regional Coalition of Rivers and Streams
The RCCRS currently uses a variety of social media platforms (Twitter, Facebook, Instagram) and
its website to educate the public on issues such as pet waste disposal, proper fertilizer and
pesticide use, proper disposal of hazardous substances such as oil and solvents, and the
connection between personal choices and water-quality in rivers and streams. They started
working with Clearing Magazine in FY2223 to promote the Honoring Our River program which is
a student art and writing program. The Honoring Our River program as reached 281,283 people.
Social media (Facebook and Instagram) reaches 305,000 people. The RCCRS continues to attract
followers on social media and saw an increase on all platforms. Has increased to 3,036
followers on Facebook, over 1,000 followers on Instagram, and 160 YouTube subscribers.
Messages included information on pet waste disposal, pesticide/herbicide use, automotive fluid
disposal, and separate versus combined stormwater systems.
Overall, the number of followers on the Coalition’s Meta accounts increased by over 100
percent on each platform. Facebook engagement decreased compared to FY 2024-2025 while
Instagram engagement increased. The reasons for the discrepancy in reach between Facebook
and Instagram was likely due to the Coalition leveraging Meta’s Advantage+ ad algorithm
feature.
Stormwater Stickers (2025)
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 22
During the FY2425 the Coalition:
•Finalized its 2024-2027 Strategic Plan.
•Increased the number of posts, including paid ads/boosted posts compared to FY 2023-2024.
•Created more original, branded content in line with the Coalition’s Social Media Content
Strategy.
•Utilized more refined and co-created seasonal content mixes.
•Gathered more insight from Coalition members to further curate and refine content.
•Developed more visually appealing ads using marketing best practices.
7.4.4 Clean Rivers Coalition
The CRC continued its award-winning “Follow the Water” video series which connects people to
their rivers, promotes actions to protect water and builds an appreciation of local water
resources. The “Follow the Water” campaign had a 77% follower growth across all platforms
during the FY2425. With 544k profile /page reach, 45k social media engagement, and 8.5k
social media followers.
FY2425, the CRC made a concerted effort to shoot, edit, and post short-form videos. CRC
posted sixteen original reels and received over 18,000 views on Facebook, with over 19 hours of
watch time on Instagram.
The CRC pesticide reduction campaign aimed at reducing pesticide and fertilizer use in FY2425.
The campaign resulted in 6.3k YouTube views, 659k Google Ads impressions, and 122.2 hrs.
YouTube Watch Time.
The level up your lawn campaign helps residents adopt water-safe lawn care practices and had
11k landing page visits, 9k YouTube views and 1,572 surveys completed.
What’s your lawn style continued its campaign which shared water safe lawn tips in English and
Spanish showcasing different lawn care styles to residents with 419K YouTube views, 10.5k
watch time and 32 new subscribers.
CRC focused on strengthen internal connections and expanding partnerships Through Honoring
Our Rivers, a statewide environmental education and arts program with poetry and artwork
honoring local watersheds. CRC also collaborated with college students at Portland State
University, linking academic learning with real-world storytelling, as well as publishing essays
that explore modern relationships with humans and water.
CRC also partnered with Changing Current, fostering a deeper collective and dialogue about
water protection with tribal communities.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 23
7.4.5 Clean Water Partners
In FY2425, PSAs by the Clean Water Partners included messages on water conservation,
herbicide and fertilizer use, car washing, fall leaf and lawn practices, and keeping leaves out of
the public stormwater system. Information regarding leaf disposal and keeping catchbasins free
of leaves were run as part of the campaign throughout the wet-weather season.
During FY2425, a total of 650 broadcasts were conducted, reaching an audience of 8,999,000
via television. Online engagement included 3,449,674 banner ad impressions on KPTV.COM,
resulting in 2,533 banner ad clicks and 3,508 page views. Social media activity consisted of eight
Facebook topic posts, generating 200,597 individual impressions and contributing to a total
reach of 12,497,322 impressions across all platforms.
7.5 PUBLIC INVOLVEMENT
Public involvement at the City includes volunteer events and public comments on projects and
documents.
7.5.1 Volunteer Events
The City works with volunteer groups
through its Friends of Parks program
and the Habitat Enhancement Program.
In FY2425, the City sponsored 9
volunteer work parties across the city to
plant approximately 3,000 native plants
and remove invasive plants.
The City reintroduced the storm drain
marking volunteer program with
updated content and promoted through
social media, with approximately 10
volunteers and 100 storm drains
marked.
The City provides yard debris bins to reduce the amount of vegetation that gets tossed into
nearby streams or raked into the street. Yard debris bins for leaf collection were provided by
the City to one neighborhood in FY2425.
Storm Drain Marking Volunteer Group (2025)
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 24
7.5.2 Public Comment
No CIP projects included public comment for FY2425.
As part of the MS4 permit requirements, staff reviewed the municipal code to determine if any
changes were needed to comply with the permit. Most of the changes were administrative in
nature and, on the direction of the City Council, staff reached out to engineers and developers
working in the city to provide comments on the proposed stormwater code amendments. Staff
received one comment. The general public was able to provide comments at the Planning
Commission and City Council hearings held in October 2024 and November 2024, respectively,
and also online through a comment form. No comments were received from the public.
As part the SWMM updates required by the MS4 permit, the City provided the document for
public comment and received one comment. The City provided articles in its newsletters,
posted on its social media, and provided an online comment form. Comments on social media
were addressed throughout FY2425.
The annual reports and adjunct documents were posted to the City's website in FY2324 and
staff created an online comment form. No comments were received regarding the annual
reports or adjunct documents in FY2425. The Industrial and Commercial Program strategy
received no comments. The Retrofit/Hydromodification update received no comments.
7.6 POLLUTION PREVENTION FOR MUNICIPAL OPERATIONS
The MS4 permit requires the City to inspect and maintain its stormwater infrastructure
including repair or replacement of non-functioning components. The City is also required to
report on street sweeping, winter maintenance, inflow/infiltration (I/I), pesticide use on City-
owned property, and emergency response.
7.6.1 Conveyance Infrastructure Maintenance
The City’s public stormwater system contains over 132 miles of stormline, 57 ditches, 300
culverts, over 4,200 catch basins, and approximately 3,000 manholes. There were no changes in
the City’s bridge inspection procedures nor to the inspection and maintenance procedures for
the stormwater conveyance system.
During FY2425, staff completed comprehensive inspection and maintenance of the stormwater
system:
• Catch basins: 1,540 units were inspected and cleaned.
• Manholes: 191 manholes were inspected and cleaned; no repairs or replacements were
required.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 25
• Stormlines: Although no stormlines required repair or replacement, 4,008 feet were
inspected and cleaned.
• UICs: 12 UICs were inspected; none required cleaning or repair.
• Ditches: 900 feet of ditches were cleaned.
• Culverts: 175 culverts were inspected and cleaned eight times throughout the year.
• Detention vaults: Four vaults were cleaned, and eight filters were replaced.
• Sediment detention ponds: Five ponds were cleaned, resulting in the removal of 45
cubic yards of debris.
Staff collected 1,107 cu yd of debris through the stormwater infrastructure maintenance
program. Approximately 1,200 cu yd was sent to the Wasco County landfill after passing a
toxicity (TCLP) analysis.
7.6.2 Street Sweeping
In general, commercial streets are swept every week while major commuter routes are swept
once a month. Curbed residential areas are swept four to six times per year. Approximately
4,630 lane miles were swept in FY2425. Debris is combined with the conveyance system debris
before being sent to the Wasco County landfill. Staff did not report any disposal to a hazardous
waste landfill.
The City’s canopy cover exceeds 50% (with an effective shade of 88.6%) which presents
challenges in regard to flooding caused by leaves blocking inlets and catch basin grates. Staff
composted approximately 800 cu yd of leaves in FY2425 as part of the street sweeping
program.
7.6.3 Winter Maintenance
Elevations in the City range from approximately 1,000 ft in the northern section (Nansen
Summit) to 98 ft at Oswego Lake in the middle of the city, to over 700 ft in the southern section
of the city (Cook’s Butte and Crestline Drive). During winter weather, these elevation changes
create challenging public safety situations. The City uses gravel and magnesium chloride to
increase safety on the streets when ice and/or snow are forecast. After a storm event, staff
sweeps the streets to remove as much sand as possible from the event.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 26
Staff laid down 120 tons of sand during
winter events. Staff worked two 12-hr
shifts in response to the winter storms
in FY2425.
Staff dispersed 6,450 gallons of
magnesium chloride during winter
events. The dispersal rate is 18 gallons
per lane mile and, while it translates to approximately 360 lane miles, many streets receive
multiple treatments each year.
7.6.4 Wastewater Infrastructure
The City videoed approximately 96,126.314 feet of wastewater lines in FY2425. Inspections and
repairs on manholes, cleanouts, and mainlines were completed at 51 locations throughout the
City at a cost of $160k. A majority of the repairs were at locations in the Oswego Lake
watershed (36 locations), (12) locations in Willamette watershed and (3) locations in Tualatin.
7.6.5 Pesticides
Parks & Recreation staff reviews the Integrated Pest Management (IPM) plan on an annual
basis. In FY2324, the IPM was changed to provide an annual report to the Parks and Recreation
Board.
Parks manages 625 acres including developed parks, athletic fields, a golf course and various
natural areas around the City. The following pesticide data also includes pesticide usage from
Public Works, the Fire Department, and the Trolley.
7.6.5.1 Pesticide Use
The City employs 16 staff and 7 contractors who are certified pesticide applicators and uses a
range of pesticides along the Willamette Trolley line, in City-owned parks and natural areas,
and in street medians. The amount of pesticides used is dependent on the efficacy of the
pesticide but also on factors that are beyond the City’s control such as the weather or seasonal
rainfall.
In FY2425, the City used approximately 140 gallons and 205 pounds of pesticides (see Table 10).
It should be noted that these amounts are the gallons and pounds of the formulations and are
not representative of the actual pesticide used since a formulation contains inert ingredients as
well as the active pesticide.
The median amount of pesticides used between FY1718 and FY2425 (7 years) is 120 gallons and
300 pounds.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 27
Table 10. Pesticides Used on City-Owned Property
Pesticide Name Active Ingredient
Quantity Used
Department
Gallons Pounds
Advion Indoxacarb 0.01 Fire
Affirm Polyoxin D Zinc Salt 3 Parks
Alpine WSG Dinotefuran 0.30 Fire
Appear II Potassium Phosphate 4 Parks
Aquaneat Glyphosate 42.91 Parks
Capstone Triclopyr and aminopyralid 0.84 Parks
Carbaryl 4L Carbaryl 1.79 Parks
Credit 5.4 Extra Glyphosate 3.75 Trolley
Dac Action Chlorothalonil Acibenzolar-s-methyl 4.8 Parks
Demand CS Lambda-cyhalothrin, 0.50 Fire
Competitor Surfactant 27.3 Parks
Heritage Azoxystrobin 2 Parks
Imazapyr
2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-
oxo-1H-imidazol-2-yl]-3-pyridinecarboxylic
acid 0.09 Parks
Instrata Propiconazole 4.7% Fluidoxinol 1.2% 2.5 Parks
MaxForce Complete
(Bayer) Hydramethylnon .008 Fire
MSM 60 Metsulfuron-methyl 0.15 Trolley
Novita Drift Control Polyvinyl Polymer (Polyacrylamide) 0.05 Trolley
Novita FoamNoMore Polydimethylsiloxane 0.05 Trolley
OptiGard Ant Gel Thiamethoxam 0.0008 Fire
Oreon Pentachloronitrobenzene 5 Parks
Rodeo Glyphosate 2.14 Parks
Speedzone
2,4-D, Mecoprop-p acid, Dicamba, and
Carfentrazone 0.39 Parks
Secure Fluazinam, Tebuconazole 0.17 Parks
Spyder Selective
Herbicide Spyder Selective Herbicide 0.47 Trolley
Sureguard Flumioxazin 0.40 Parks
Traction Fluazinam 0.46 Parks
Transline Clopyralid 0.07 Parks
Turfcide Pentochloronitrobenzene 40% 200 Parks
Vastlan Triclopyr 36.81 Parks
Whetstone Aminopyralid 1.09 Trolley
Total 139.65 205
1 – P=Parks and Recreation, E=Engineering, PW=Public Works, F=Fire
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 28
7.6.5.2 Reduction in Pesticide Use
Staff use a combination of mechanical, biological,
and chemical methods to reduce pesticide use in
City-managed vegetative areas. Diseased, dead, or
damaged plants are removed by hand while
pruning at the right time promotes air circulation
for healthier plant growth. Hand weeding as well
as vinegar solutions are used to keep weeds and
invasive species out of landscaping. Mulch is used
to retain moisture and reduce weed growth and
rodent damage. Grass seed mixes, an increase in
turf height, and aeration reduce weeds and
irrigation needs as well as providing optimum growth opportunities for turf. Three of the City’s
properties (Greentree Park, Rossman Park, and the Adult Community Center) are managed
without any pesticides.
As another avenue for reducing pesticides, the City became a Bee City in September 2023 which
includes a commitment to 1) reducing pesticide use, 2) creating or enhancing pollinator habitat,
3) incorporating pollinator-conscious practices into city policies, and 4) hosting pollinator-
awareness events. The City created a pollinator garden at Iron Mountain Park that received a
platinum designation from the Backyard Habitat program.
7.7 STORMWATER FACILITY MAINTENANCE
The City inspects and maintains public stormwater facilities and requires private owners to
maintain private facilities. Public stormwater facilities are added to the GIS (LO Maps) when as-
builts are submitted at project completion.
7.7.1 Public Stormwater Facilities
There were no changes to the inspection or maintenance procedures for public stormwater
facilities. Public stormwater facility maintenance by Parks staff primarily consists of removal of
invasive species and debris. Public Works facilities, such as filter vaults and vegetated
stormwater facilities, are maintained by staff and a contractor.
In FY2425, four water-quality vaults were inspected and cleaned with eight cartridges replaced
in the vaults. City staff and contractors maintain approximately 140 vegetated stormwater
facilities located throughout the city with a majority of the facilities located in the Oswego Lake
watershed (see Table 11). In FY2425, staff and contractors replaced dead trees in planters
along Boones Ferry Rd, dead plants and trees in planters along D Avenue and Kerr Pkwy.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 29
7.7.2 Private Stormwater Facilities
In FY1819, the City updated its policies and procedures and began routine inspections of private
stormwater facilities in FY1920. The City has placed a higher priority on educating property
owners about their stormwater facilities and keeping the most recently completed facilities
functioning properly.
In FY2425, staff inspected 10 private stormwater
facilities and worked with property owners to
provide education on the proper operation of
stormwater systems. The most common issues
during inspections observed included rain gardens
and shared bioswales that required replanting and
removal of invasive vegetation or UICs that needed
cleaning. Staff ensured that each property owner
received an operations and maintenance checklist,
if they did not already have one, as well as a City-
approved plant list.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 30
Table 11. Public Vegetated Stormwater Facilities
Location Watershed Type
Facilities
Inspected/
Maintained Restoration
Public Works Facilities
Meadowcreek Ball Creek Planters 4 N
Westlake Ball Creek Planters 2 N
Boone's Ferry Oswego Lake Planters 43 Y (replaced dead trees)
West Bay/Virginia Oswego Lake Planters 14 N
Second Street Oswego Lake Planters 10 N
Leonard Street Oswego Lake Planter 1 N
Jean Rd/Bryant Oswego Lake Planters 11 N
Jean/Lakeview Oswego Lake Detention Pond 1 N
Pilkington Oswego Lake Detention Pond 1 N
Maintenance Center Oswego Lake Bioswales 3 N
Knaus Road Springbrook Bioswales 4 N
Kerr Pkwy Springbrook Planters 2 Y (added plants)
D Ave Tryon Creek Planters 29
Y (replaced dead plants,
added trees)
Tenth St Tryon Creek Planters 8 N
Park and Recreation Facilities
Luscher Farms Oswego Lake Detention Pond 1 N
Hazelia Park Oswego Lake Swale 1 N
Sundeleaf Plaza Oswego Lake Raingardens 3 N
Iron Mtn Park Springbrook Raingardens 2 N
Brock Property Tualatin River Swale 1 N
8 ENVIRONMENTAL MONITORING PROGRAM
The City’s monitoring plan includes monthly sampling at six sites and stormwater sampling at
two sites (see Figure 2). The City also maintains continuous monitoring (gaging) stations at the
mouths of Lost Dog Creek and Springbrook Creek. Instrumentation at the gaging stations collect
discharge and field data in 15-minute increments.
The City provided $2,200 to the US Geological Survey (USGS) in FY2425 to operate and maintain
the USGS hydrological station (#14207500 - Tualatin River @ West Linn). The amount is the
City’s share of a collaborative effort with other co-permittees.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 31
Figure 2. Wet Weather and In-Stream Monitoring Sites in Lake Oswego
8.1 MONITORING PLAN CHANGES
The City was required to update its monitoring plan as part of the 2021 MS4 permit and
received DEQ approval of the revised plan in June 2023. The new plan revised the stormwater
monitoring locations and added pesticides as a regular monitoring activity for stormwater
sampling. Sampling changes included:
• Removal of biological oxygen demand and turbidity
• Removal of total organic carbon from quarterly sampling
• Addition of total iron to quarterly sampling
• Addition of field blanks for quality control
• Dissolved organic carbon was changed from quarterly to monthly sampling;
• Volatile suspended solids (VSS) were changed from monthly to quarterly sampling
While not required by the permit, VSS was kept in the instream sampling suite because of the
dissolved oxygen TMDLs for Fanno Creek and its tributaries. Iron was added to the instream
sampling suites because of its 303(d) listing in the Tualatin River basin and the historical iron
mining in the area.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 32
Mercury sampling, required as part of the 2021 MS4 permit, will be completed by staff from
Water Environment Services in a collaborative effort by the co-permittees. The City is providing
two quarterly monitoring sites, located at Nettle Creek and Carter Creek, for the effort.
8.2 WET WEATHER MONITORING
In FY2324, the City moved its wet weather monitoring sites to one in the Carter Creek (Tualatin
River) watershed and one in the Oswego Lake watershed. The Oswego Lake watershed is
downgradient from the previous Reese Rd site but collects more runoff from residential
properties.
The new wet weather site on Reese Road is in a commercial and residential area. It receives
runoff from Boones Ferry Rd, a major arterial street in the City, which finished a major
stormwater renovation in FY2122 that included the construction of planters throughout the
project. The wet weather site on Meadows Rd is primarily non-SFR with multi-family residential,
office buildings, and restaurants in its watershed.
Stormwater is collected during rain events that provide at least 0.1 inches of precipitation.
Water samples are collected at each site for a variety of parameters such as metals, nutrients,
hardness, and bacteria. Field measurements include pH, dissolved oxygen, temperature, and
conductivity. Starting in FY2324, pesticides were also collected at the sites.
In FY2425, stormwater samples were collected during storms that produced 0.29 inches and
0.12 inches of precipitation. In 2024, 2,4-D was detected at the Carter site at a concentration of
0.11 µg/L, and at the Reese site at 0.19 µg/L, where Bifenthrin was also detected at 0.092 µg/L.
In 2025, 2,4-D was again detected at the Reese site at a concentration of 0.12 µg/L. There were
no detections in FY2324 so the data trends and analysis will be compared in FY2526.
8.3 INSTREAM MONITORING
The City collects monthly instream water samples from six sites. Three of the sites discharge to
Oswego Lake, two discharge to Fanno Creek (a Tualatin River tributary), and one discharges to
Tryon Creek (a Willamette River tributary).
Water samples are collected at each site for a variety of parameters such as metals, nutrients,
hardness, and bacteria. Each quarter, the City collects information on organic carbon, cations,
and anions. Field measurements include pH, dissolved oxygen, stream temperature, and
conductivity. The data collected at these sites is discussed in more detail with the discussion on
TMDLs and water-quality limited streams (Sections 8.6 and 8.7, respectively).
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 33
8.4 CONTINUOUS MONITORING STATIONS
The City upgraded the equipment at its continuous monitoring stations in 2018 and 2019. In
2018, both stations received new submersible pressure transducers to measure stage and new
YSI EXO-3 sondes to measure dissolved oxygen, turbidity, pH, conductivity, and temperature.
Crest-stage gages were installed in Springbrook Creek in 2018 and in Lost Dog Creek in 2019 to
verify peak flows. A new staff gage was installed in October 2019 at Lost Dog Creek and
relocated downstream of the original gage to improve flow data measurements.
Gaging station data is analyzed by water year (WY); the following discussion is related to water
quality and discharge data collected during WY23 (October 2023 to September 2024).
8.4.1 Lost Dog Creek
Lost Dog Creek drains an urbanized area of 0.66 sq mi with an average slope of 7%. It is
composed of three branches which start at Bergis Rd, Luscher Farms, and the Lake Oswego
Public Golf Course. Two branches join within the first 1,300 ft but the third branch joins
approximately 500 ft before Lost Dog discharges into Oswego Lake. The Bergis Rd and Luscher
Farms branches daylight in and out of piped areas that are either undeveloped or have
residential and institutional uses (churches, golf courses, and schools).
The stream channel near the gaging station is moderately entrenched with cascading pools,
steep banks, and little vegetation in the channel. The streambed near the gaging station is
composed primarily of gravel and cobbles with some boulders.
Discharge during WY24 ranged from 0.07 ft3/s to an estimated 11.93 ft3/s. The lowest
discharge happened on August 24, 2024. The maximum discharge occurred on December 5th,
2023.
The median water temperature for WY24 was 11.8°C with a maximum reached of 22.1°C during
a rain event in the late afternoon of August 17th during a low flow of 0.06 cfs. Dissolved oxygen
concentrations averaged 11.01 mg/L with a minimum of 8.1 mg/L on August 2nd.
The pH module was replaced on August 21, 2024 due to values starting to rise in late June. A
calibration determined that the recorded slope was poor and that the pH module needed
replacement. After August 21, 2024, the data improved. The average pH was 7.62 for the WY24.
8.4.2 Springbrook Creek
Springbrook Creek drains an urbanized area of 1.98 sq mi with an average slope of 4.5%. It is
largely unconfined and composed of approximately nine branches, three of which drain a
majority of the watershed which contains a mixture of uses.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 34
The stream channel in the area of the gaging station is trapezoidal with steep banks and very
little vegetation in the channel. The streambed is composed of fines and gravel with cobbles
and boulders on top.
The 2024 water quality record was influenced by upstream activities from beavers building
dams (approximately 200 ft upstream of the gage) and the City of Lake Oswego Parks and
Recreation removing the beaver dams. A beaver deceiver pipe was installed in November of
2023 according to the City. Installation of beaver deceiver was noticed during December 15,
2023, site visit by WEST. The beaver deceiver appeared to lose some amount of functionality
near the end of March 2024. The water quality parameters show more frequent spikes that are
likely caused by manual removal of debris upstream as they do not follow precipitation trends
or match the patterns of nearby Lost Dog Creek. The deceiver was replaced by city employees
on November 19, 2024.
Discharge during WY24 ranged from 0.5 ft3/s to an estimated 26.53 ft3/s. The lowest discharge
happened on August 21, 2024. The maximum discharge occurred on December 5th, 2023.
The median water temperature for WY24 was 12.5°C with a maximum reached of 20.3°C on
July 9, 2024. Dissolved oxygen concentrations averaged 8.76 mg/L with a minimum of -0.1 mg/L
on February 9th 2024. The average pH was 6.56. The highest pH was recorded on February 23,
2024 at 7.4 and lowest of 6.2 pH on February 10, 2024.
8.5 MACROINVERTEBRATE ASSESSMENT
To assess biological communities and the long-term water-quality of its streams, the City
partners with Clackamas County co-permittees to conduct an assessment every 3 to 4 years
with the latest assessment completed in January 2025.
The City began biological sampling in 2004 with six perennial stream reaches. In 2007, five
additional reaches were added and one site was dropped. These 10 reaches were then re-
sampled in 2007, 2009, 2013, 2018, and 2021.
8.5.1 Results
Results were analyzed using the multi-metric macroinvertebrate-based index of biological
integrity (DEQ, 2003) and the Predictive Assessment Tool for Oregon (Hubler, 2008). The multi-
metric macroinvertebrate-based index of biological integrity (M-IBI) is a matrix of ten attributes
that have been shown to have an effect on the macroinvertebrate community such as the
number and types of sensitive taxa present. The Predictive Assessment Tool for Oregon
(PREDATOR) is based on the observed populations at the sample site versus the expected
populations based on a relatively pristine environment (Marine Western Coastal Forest).
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 35
Table 12. Macroinvertebrate Sampling Sites
Name Latitude Longitude Watershed
Land Use Assessments
NA R MF I C
First
Year
Times
Assessed
Nettle Creek
at Atwater Rd 45.4246 -122.6814 Tryon Creek X 2004 7
Tryon Creek
abv State St 45.4243 -122.6613 Tryon Creek X X X X X 2007 6
Springbrook
abv Boones Way 45.4140 -122.7151
Springbrook/
Oswego Lake X X X X X 2004 7
Springbrook
at Iron Mtn1 45.4142 -122.7078
Springbrook/
Oswego Lake X X X X X 2004 7
Lost Dog
nr Aspen Ct 45.4025 -122.6806 Oswego Lake X X 2007 5
Lost Dog at
Municipal Golf Course 45.4006 -122.6891 Oswego Lake X X 2004 7
Lost Dog
at Lake Front1 45.4100 -122.6782 Oswego Lake X X 2004 7
Oswego Creek
bel State St 45.4107 -122.6625 Oswego Lake X X X 2007 6
Ball Creek
bel Kruse Oaks1 45.4245 -122.7403 Tualatin River X X X 2007 6
Carter Creek
abv Bangy1 45.4170 -122.7403 Tualatin River X X X 2007 6
1 – At or near instream monitoring site
NA – Natural Area, R – Residential, MF – Muti-Family Residential, I – Institutional, C - Commercial
8.5.1.1 PREDATOR Scores
As shown in Table 13, all ten sites were classified as severely impaired under the PREDATOR
method, consistent with results from 2021. However, Tryon Creek and Ball Creek showed slight
improvement, while Springbrook above Boones Way, Lost Dog at Lake Front, and Oswego Creek
showed signs of decline. Conditions at Springbrook at Iron Mountain, Lost Dog at the golf
course, and Carter Creek remained unchanged. Lost Dog near Aspen could not be compared to
2021 results due to dry conditions.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 36
8.5.1.2 M-IBI Scores
M-IBI scores indicated better biological conditions compared to the O/E score for the same site
at four of the 10 sites sampled. Ball Creek had the highest M-IBI score in 2024, while Tryon
Creek had the highest O/E score.
All 2024 sampling sites received O/E scores indicating poor biological condition, while six sites
received M-IBI scores indicating severe impairment. Comparison of 2021 and 2024 model
scores showed little change. M-IBI scores were higher in 2024 at three sites, but only one site
Nettle Creek moved from severely impaired to moderately impaired as a result. In addition, M-
IBI scores were the same in 2021 and 2024 at four sites. O/E scores were the same in both years
at three sites, and the small magnitude of difference in O/E scores at the remaining sites
suggests little change in habitat.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 37
Table 13. Physical and Biological Characteristics of Selected Streams in Lake Oswego, 2024
Site
PREDATOR Score1 M-IBI Score2
Temperature Score
°C
Fine Sediment Score
%
Significant
Unidirectional
Habitat Trends
2013 2018 2021
2024 2013 2018 2021
2024 2013 2018 2021
2024 2013 2018 2021
2024 2024
Nettle Creek
at Atwater Rd 0.34 0.44 0.48
0.48 18 20 18
22 16.8 17.3 16.7
17.0 22 25 15.
19 None
Tryon Creek
abv State St 0.48 0.39 0.44
0.53 24 22 20
20 21.2 23.0 22.3
23.1 18 19 18
19 DEC5: # EPT6 taxa
Springbrook
abv Boones
Way 0.39 0.58 0.53
0.44 20 18 16
16 21.2 21.7 22.7
22.8 37 36 36
32 None
Springbrook at
Iron Mtn3 0.44 0.49 0.39
0.39 16 18 12
14 22.1 22.1 24.0
20.6 39 30 32
26 None
Lost Dog nr
Aspen Ct4 0.44 0.34 NA
0.49 18 18 NA
22 16.8 17.0 NA
18.2 50 37 NA
29 INC7: M-IBI
Lost Dog at
Municipal Golf
Course 0.24 0.29 0.19
0.19 10 18 14
18 19.3 17.9 20.2
20.8 43 30 48
78
INC: Sediment
Stressor Score
Lost Dog at
Lake Front3 0.29 0.24 0.44
0.24 16 14 20
14 15.9 16.9 17.7
19.1 32 26 22
70 None
Oswego Creek
bel State St 0.19 0.29 0.34
0.29 14 16 16
16 26.3 25.9 24.0
25.2 54 45 40
42 None
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 38
Site
PREDATOR Score1 M-IBI Score2
Temperature Score
°C
Fine Sediment Score
%
Significant
Unidirectional
Habitat Trends
2013 2018 2021
2024 2013 2018 2021
2024 2013 2018 2021
2024 2013 2018 2021
2024 2024
Ball Creek bel
Kruse Oaks3 0.44 0.39 0.39
0.44 18 18 24
24 20.6 19.4 19.7
17.9 43 30 27
16
INC: O/E score, #
EPT taxa
Carter Creek
abv Bangy3 0.39 0.29 0.34
0.34 16 18 18
16 21.9 20.8 20.8
21.1 NA 42 44
51 None
1 – Scores range from <0.75 (severe impairment) to >0.90 (no impairment). 2 – Scores range from < 20 (severe impairment) to >39 (no impairment 3 – At or near an instream
monitoring site; 4 – Not included in the 2021 assessment due to lack of water; 5 – Decrease 6 – EPT (Ephemeroptera [mayfly], Plecoptera [stonefly], Trichoptera [caddisfly]) taxa
7 – Increase
PREDATOR=Predictive Assessment Tool for Oregon; M-IBI=Multimetric Macroinvertebrate-based Index of Biological Integrity; °C=degrees Celsius
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 39
8.5.2 Stressors
Temperature stressor scores at three sites in 2024 were below the threshold value at which
temperature is considered a potential stressor (18.4ºC); two sites scored below this threshold
in 2021.
Two sites had sediment stressor scores below the 19% threshold in 2021; in 2024, no sites
scored below this threshold.
No significant unidirectional trend in number of total taxa or temperature stressor score was
seen at any Lake Oswego site. Only four sites had significant trends in any metric, with no more
than two metrics changing significantly over time at any site.
8.6 TOTAL MAXIMUM DAILY LOADS
Within the City’s MS4 jurisdiction are several streams, monitored and unmonitored, that are
affected by Total Maximum Daily Loads (TMDLs). All of the TMDLs, except for temperature,
have a wasteload allocation (WLA). For the purposes of this report, the following discussion
focuses on TMDLs affecting streams that are part of the City’s instream monitoring program in
order to provide adequate data for the discussion. The discussion uses medians, as opposed to
averages, because they are less sensitive to outliers. Geomeans are used for bacterial analyses
because of the volatility of the results even between two replicate samples.
8.6.1 Bacteria
Bacterial sources include animal waste, wastewater system overflows, failing septic systems,
and illegal dumping. For the City, bacterial TMDLs have been set for the Willamette River, the
Tualatin River (all tributaries and ephemeral streams), and Springbrook Creek (see Table 14).
Table 14. Bacterial Wasteload Allocations for Streams in Lake Oswego
Waterbody Affected Stream
Parameter
and Year Season/Target Wasteload Allocation
Tualatin River1
(all streams)
Carter Creek
Ball Creek
Bacteria
(2001)
Maximum
Summer Stormwater
Winter Stormwater
406 Col/100 mL
12,000 Col/100 mL
5,000 Col/100 mL
Springbrook
Creek2 Springbrook Creek
Bacteria
(2006)
Maximum
80% Reduction3
406 Col/100 mL
Winter Geomean/Median: 208/172
Winter Min/Max: 13/2,419
Summer Geomean/Median: 365/388
Summer Min/Max: 34/2,420
Willamette
River2 (all
tributaries)
Nettle Creek
Lost Dog Creek
Boones Ferry
Bacteria
(2006)
Maximum
78% Reduction4
406 Col/100 mL
Dependent on Stream
1- Summer=May 1-Oct 31, Winter=Nov 1 to Apr 30; 2 – Summer=June 1-Sept 30, Winter=Oct 1 to May 31; 3 – Based on 1997-
2003 data; 4 – Based on 1996-2002 data
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 40
8.6.1.1 Tualatin River
Monitoring sites affected by the Tualatin River bacteria TMDL are Carter Creek and Ball Creek
(see Table 15). Geomean concentrations for Ball Creek and Carter Creek were below the 406
col/100 mL threshold for the winter and summer, Ball creek’s winter and summer geomean fell
below the threshold while Carter Creek was slightly above for the winter geomean but fell
under the geomean for summer.
Table 15. E. Coli Concentrations in Ball Creek and Carter Creek.
Threshold
Ball Creek Carter Creek
1997-2003
(Col/100mL)
FY1718-FY2122
(Col/100mL)
FY2425
(Col/100mL) 1997-2003
(Col/100mL)
FY1718-FY2122
(Col/100 mL)
FY2425
(Col/100mL)
Winter
Maximum 2,300 >2,420 228 >2,420 >2,420 210
Winter
Geomean 190 96 48 277 89 96
Summer
Maximum >2,420 >2,420 1088 2,200 1,200 201
Summer
Geomean 418 283 151 119 163 97
8.6.1.2 Springbrook Creek
Based on data collected by the City from 1997 to 2003, DEQ determined that bacterial
concentrations in Springbrook Creek must be reduced to 80% of their 1997 to 2003
concentrations (see Table 16).
The bacterial data indicates that E. coli levels have decreased compared to FY2324. Two months
of data from FY2324 were missing, and one month recorded levels at or above 2,420/mL due to
an ongoing SSO event, there was also another SSO in the Springbrook watershed in FY2425.
Although a reduction was observed, the targeted geometric mean threshold was not achieved
for FY2425.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 41
Table 16. E Coli. Concentrations in Springbrook Creek.
Threshold
1997-2003
Col/100mL)
80% Reduction
Target
(Col/100 mL)
Historical1
Concentration
(Col/100mL)
FY2425
(Col/100 mL)
Summer Maximum 2,420 484 >2,420 866
Summer Geomean 365 73 133 180
Summer Minimum 34 7 19 63
Winter Maximum 2,419 484 >2,420 980
Winter Geomean 208 42 145 100
Winter Minimum 13 3 10 29
1 - Historical is from FY1718 to FY2122
8.6.1.3 Willamette River Tributaries
One monitoring site (Nettle Creek) is in the Tryon Creek watershed. Two other monitoring sites
(Boones Ferry @ Bryant and Lost Dog Creek) discharge to Oswego Lake which should act as a
settling area for loads coming from those sites.
The Willamette River TMDL for bacteria requires a 78% reduction of the E. Coli concentrations
from 1997 to 2002. As shown in Table 17, the E. Coli geomean concentration show reductions
of 14% for Nettle Creek, 70% for Lost Dog, and 42% for Boones Ferry. Reductions in the
maximum concentrations were 62% for Nettle Creek, 86% at Lost Dog, and 76% at Boones
Ferry.
Table 17. E. Coli concentrations in Nettle Creek, Lost Dog Creek, and the Boones Ferry sampling site.
Threshold
Nettle Creek Lost Dog Boones Ferry
1997-
2002
78%
Reduction
Target
(E. Coli)
FY2425 1997-
2002
78%
Reduction
Target
(E. Coli)
FY2425 1997-
2002
78%
Reduction
Target
(E. Coli)
FY2425
Maximum >2,420 532 921 2,420 532 328 2,420 532 579
Geomean 297 65 256 144 32 43 84 18.5 49
Minimum 21 4.6 88 2 0.44 2 1 0.22 6
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 42
8.6.2 Dissolved Oxygen
The 2001 dissolved oxygen (DO) WLA for Fanno Creek and its tributaries required a minimum
concentration of 6.5 mg/L and a 50% decrease from pre-1998 concentrations for total
suspended solids (TSS) and total volatile suspended solids (VSS) from May 1st to October 31st.
Because the City’s monitoring data starts in 1997, the following analysis assumes the baseline
TSS conditions are from July 1997 to June 1999. VSS baseline data is the first two years of
monitoring (August 2012 to July 2014).
Ball Creek and Carter Creek, as tributaries of Fanno Creek, are covered under the Fanno Creek’s
WLA. The minimum concentration of 6.5 mg/L for DO was met for Carter Creek but not for Ball
Creek in FY2425(see Table 18). A measure of the longer-term DO concentration experienced by
macroinvertebrates is the median DO concentration. The threshold concentration of 6.5 mg/L
for the median DO concentration was not met in both Ball Creek and Carter Creek for FY2425,
however there was an overall reduction for both minimum and median from both the baseline
and historical time periods.
Ball Creek saw a 94% reduction in the maximum TSS concentration and a 50% reduction in the
median TSS concentration meeting its reduction targets. The maximum and median VSS
concentrations also met the reduction target of 50%.
Carter Creek saw an 90% reduction in TSS from the baseline concentrations and a 47%
reduction in median concentrations not meeting the reduction target of 50% for median. The
reduction for VSS concentrations at Carter Creek was 94% for maximum concentrations and
94% for median concentrations.
Table 18. Dissolved Oxygen, TSS, and VSS concentrations in Ball Creek and Carter Creek
Time Period
DO, mg/L TSS, mg/L VSS, mg/L
Minimum Median Maximum Median Maximum Median
Ball Creek
Baseline1 7.9 9.8 100.0 7.0 57 41
FY1718-FY2122 8.7 9.5 21.5 3.0 80 34
FY2425 7.0 9.3 6.0 3.5 4.0 3.5
Change2 -11% -5% -94% -50% -93% -91%
Carter Creek
Baseline1 5.4 8.6 80.0 9.5 70 45
FY1718-FY2122 4.5 7.0 20.0 5.0 58 37
FY2425 4.8 8.1 8.0 5.0 4.0 2.8
Change2 -11% -6% -90% -47% -94% -94%
1 – Baseline is Jul 1997-Jun 1999 for TSS and DO and, for VSS, from Aug 2012 to Jul 2014; 2 - % change is calculated between
baseline and FY2 425 concentrations
DO=dissolved oxygen; TSS=total suspended solids; VSS=volatile suspended solids
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 43
8.6.3 Chlorophyll α and pH
Chlorophyll α and pH TMDLs were set in the 2001 Tualatin River TMDL and revisited, in 2012,
for Oswego Lake and its tributaries (see Table 19). Total phosphorous is used as a surrogate
measure for pH and chlorophyll α. Monitoring sites affected by the TMDL include Ball Creek,
Carter Creek, Springbrook Creek, Lost Dog Creek, and Boones Ferry at Bryant.
Table 19. Chlorophyll α and pH Wasteload Allocations for Streams in Lake Oswego
Waterbody
Applicable
Lake Oswego
Monitoring Sites Parameter1 Season/Target2
Wasteload
Allocation
Fanno Creek
(all tributaries)
Carter Creek
Ball Creek
pH (2001)
Clorophyll α (2001)
Summer Median Phosphorous
Summer Max Phosphorous
pH Range
0.13 mg/L
0.46 mg/L
6.5 – 8.5
Tualatin River
(all tributaries
and ephemeral
streams)
No monitoring
sites
pH (2001)
Clorophyll α (2001)
Summer Median Phosphorous
Summer Max Phosphorous
pH Range
0.14 mg/L
0.49 mg/L
6.5 – 8.5
Oswego Lake
(all tributaries
and ephemeral
streams)
Springbrook
Creek
Lost Dog Creek
Boones Ferry
Total phosphorous
(2001 and 2012)
Summer Baseflow Conc
Summer Baseflow Load
Summer Stormwater Load
Winter Baseflow Conc
Winter Baseflow Load
Winter Stormwater Load
0.09 mg/L
272 lbs/season
139 lbs/season
0.15 mg/L
986 lbs/season
858 lbs/season
8.6.3.1 Fanno Creek Tributaries
The maximum summer phosphorous concentration for the Tualatin River watershed was
determined by DEQ to be 0.46 mg/L with a median of 0.13 mg/L (see Table 20).
Table 20. Total Phosphorous Concentrations and pH at Ball Creek and Carter Creek
pH Range,
Summer
Total Phosphorous,
Summer Maximum
(mg/L)
Total Phosphorous,
Summer Median
(mg/L)
WLA 6.5 – 8.5 0.46 0.13
Ball Creek
Historical (FY1718-FY2122) 7.0 – 8.4 0.23 0.10
FY2425 6.5 – 7.9 0.12 0.09
Carter Creek
Historical (FY1718-FY2122) 6.4 – 8.2 0.50 0.12
FY2425 7.0 – 7.3 0.18 0.17
1 – Summer is from May 1 to October 31
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 44
Ball Creek met the pH range and was below the median and maximum phosphorous WLAs. Ball
Creek has seen a 48% reduction in the maximum summer phosphorous concentration from
historical levels.
Carter Creek met the pH range and was below the maximum phosphorous WLA but slightly
above the median phosphorous WLAs. Carter Creek has seen a 64% reduction from historical
concentrations for the maximum summer phosphorous concentration.
8.6.3.2 Oswego Lake
The maximum summer concentration for total phosphorous in the Oswego Lake watershed was
set by DEQ in 2001 to 0.09 mg/L with a maximum winter concentration of 0.15 mg/L (see Table
21). Monitoring sites within the Oswego Lake watershed include Springbrook Creek, Lost Dog
Creek, and the Boones Ferry at Bryant location.
Lost Dog Creek monitoring sites met the phosphorous WLA for the summer and winter
maximums as well as the pH range requirements. The summer maximum concentration shows
a 40% reduction from the historical levels. The winter maximum concentration shows a slight
increase from the historical levels.
Springbrook Creek was above the WLA for the summer and below for winter maximum
concentrations. The summer maximum concentration shows a 46% reduction and the winter
maximum concentration shows a 78% reduction from the historical levels. The pH range
requirements were not met, they were slightly below the low end pf the pH range of 6.5.
Boones Ferry was above the summer maximum but below the winter maximum concentrations.
The summer maximum shows a 31% reduction and the winter maximum concentration shows a
33% reduction from the historical levels. The pH range requirements were met.
Table 21. Total Phosphorous Concentrations and pH in the Oswego Lake Watershed
pH
Range
Total Phosphorous,
Summer Maximum (mg/L)
Total Phosphorous,
Winter Maximum (mg/L)
WLA 6.5 – 8.5 0.09 0.15
Lost Dog Creek
Historical (FY1718 -FY2122) 6.7 – 7.9 0.10 0.08
FY2425 7.4 – 7.8 0.06 0.09
Springbrook Creek
Historical (FY1718 -FY2122) 6.3 – 8.0 0.26 0.64
FY2425 6.3 – 6.8 0.14 0.14
Boones Ferry at Bryant
Historical (FY1718 -FY2122) 6.3 – 8.1 0.16 0.15
FY2425 6.5 – 6.9 0.11 0.10
1 – Summer is from May 1 to October 31; 2 – Winter is from November 1 to April 30
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 45
8.6.4 Biocriteria
The threshold for biocriteria is a sufficient (water) quality to support aquatic species without
detrimental changes in the resident biological community (DEQ, 2022). Water bodies that
have biocriteria impairment include Fanno Creek and Tryon Creek. Tributaries to these streams
that are within the City’s jurisdiction include Ball Creek, Carter Creek, and Nettle Creek. All
three are part of the City’s instream monitoring network.
The City partnered with Water Environment Services, a co-permittee, to complete a
macroinvertebrate assessment in 2024. A summary of those results for Ball Creek, Carter Creek,
and Nettle Creek is available in Section 8.5.
8.7 WATER-QUALITY LIMITED STREAMS
Sections 303(d) and 305(b) of the CWA requires DEQ to determine if a waterbody contains
pollutants at levels that exceed protective water-quality standards. DEQ uses water-quality data
to evaluate the most common beneficial uses such as aquatic life, drinking water, and
recreation. Waterbodies that exceed the water-quality standards are identified as impaired and
prioritizes the waterbody for the development of a TMDL.
The current Integrated Report, commonly referred to as the “303(d) list”, was approved by EPA
in September 2022. While only Oswego Lake and Oswego Creek were specifically listed in the
2022 assessment, an impaired stream may include all of the tributaries to the stream or the City
may have public outfalls discharging to the affected stream segment (see Table 22).
Table 22. Designated 303(d) Streams and Tributaries, 2022
Stream
Affected
Stream Reach
Tributary
Stream Parameter Time Period Threshold
Fanno
Creek
Carter Creek to
Tualatin River Carter Creek
Dieldrin1
TTCE1 Not Specified
Table 40,
OAR 340-041-8033
Fanno
Creek
Carter Creek to
Tualatin River Carter Creek
Total Iron1
Copper Not Specified
Table 30,
OAR 340-041-8033
Fanno
Creek
Carter Creek to
Tualatin River Carter Creek Dissolved Oxygen
Spawning
(Cool Water)
Fanno
Creek
1st through 4th
order streams
Carter Creek
Ball Creek Dissolved Oxygen
Spawning
(Cool/Cold
Water)
Fanno
Creek
1st through 4th
order streams
Carter Creek
Ball Creek
Biocriteria, Aquatic Weeds4
Chromium VI1, Zinc3 Not Specified
Table 30,
OAR 340-041-8033
(Cr and Zn only)
Oswego
Lake Lake
Lost Dog Creek
Springbrook Creek
Boones Ferry Algal Blooms Not Specified
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 46
Stream
Affected
Stream Reach
Tributary
Stream Parameter Time Period Threshold
Oswego
Creek
1st through 4th
order streams
Lost Dog Creek
Springbrook Creek
Boones Ferry Temperature3
Spawning
Year Round
Oswego
Creek
1st through 4th
order streams
Lost Dog Creek
Springbrook Creek
Boones Ferry
Biocriteria
Zinc3
Copper2,4
Spawning
Year Round
Table 30,
OAR 340-041-8033
(Zn and Cu only)
Oswego
Creek
1st through 4th
order streams
Lost Dog Creek
Springbrook Creek
Boones Ferry
Benz(a)anthracene1,4
Benzo(a)pyrene1,4
Benzo(b)fluoranthene 3,41,4
Benzo(k)fluoranthene1,4
Chrysene1,4
Indeno(1,2,3-cd)pyrene1,4 Not Specified
Table 40, OARS 340-
041-8033
Tryon
Creek
Palantine Hill
Creek to
Willamette River Nettle Creek Dissolved Oxygen
Spawning
Oct 15–May
15
DO > 9 mg/L
(OAR 340-041-016)
Tualatin
River
McFee Creek to
Willamette River
Streams on City’s
south side Dissolved Oxygen3
Spawning
(Cool Water)
Tualatin
River
McFee Creek to
Willamette River
Streams on City’s
south side
Biocriteria, Harmful Algal
Blooms, Total Iron1 Not Specified
Table 30,
OAR 340-041-8033
(Iron Only)
Tualatin
River
McFee Creek to
Willamette River
Streams on City’s
south side Dieldrin1,3 Not Specified
Table 40,
OAR 340-041-8033
Willamette
River
Clackamas River
to Johnson Creek
Streams on City’s
east side
Biocriteria; Cyanide1;
Temperature
All Year
(Temp)
Table 30,
OAR 340-041-8033
(Cyanide Only)
Willamette
River
Clackamas River
to Johnson Creek
Streams on City’s
east side
Ethylbenzene1, PAHs1, PCBs1;
Hexachlorobenzene1; Aldrin1;
Dieldrin1; DDE 4,41; DDT 4,41 Not Specified
Table 40,
OAR 340-041-8033
1 – No Monitoring Requirement; 2 - Samples were mostly in the Tryon Creek watershed; 3 – New for 2022; 4 – New for 2018
8.7.1 Algal Blooms and Aquatic Weeds
Algal blooms were listed for the Tualatin River (from McFee Creek to the Willamette River) and
Oswego Lake. Aquatic weeds were listed for Fanno Creek and its tributaries. The criteria used to
list algal blooms and aquatic weeds is the statewide prohibition on “deleterious or injurious
effects on aquatic and human beneficial uses from biological growths” (DEQ, 2022).
Streams that may be affected by the 303(d) listing include Carter Creek and Ball Creek in the
Fanno Creek watershed, tributaries to Oswego Lake, and Tualatin River tributaries on the south
side of the City. Instream monitoring locations providing data for this listing include Lost Dog
Creek at Lake Front, Springbrook Creek at Iron Mountain, Boones Ferry at Bryant, Ball Creek bel
Kruse Oaks, and Carter Creek abv Bangy.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 47
Algal blooms and aquatic weeds may be caused, in part, by elevated nutrients with
phosphorous known as the limiting factor for plant growth. There are currently TMDLs in place
for the tributaries of Fanno Creek and Oswego Lake and the discussion of these TMDLs is found
in Section 8.6.
8.7.2 Biocriteria
OAR 340-041-0011 specifically addresses biocriteria: “Waters of the State must be of sufficient
quality to support aquatic species without detrimental changes in the resident biological
communities.” (DEQ, 2022). Biological monitoring is required by the MS4 permit at least once
per permit term and is generally conducted by the City every 3-4 years.
Biocriteria was listed for Fanno Creek and its tributaries, Oswego Creek and its tributaries, the
Tualatin River (from McFee Creek to the Willamette River), and the Willamette River (from the
Clackamas River to Johnson Creek). Streams that may be affected by this listing include streams
on the City’s south side, Carter Creek, and Ball Creek in the Fanno Creek watershed; Lost Dog
Creek and Springbrook Creek in the Oswego Creek/Oswego Lake watershed; and Nettle Creek
and streams on the City’s east side. All ten of the City’s macroinvertebrate monitoring sites
could be affected by these listings.
In the 2024, the City partnered with Water Environment Services to conduct a
macroinvertebrate assessment which included the City’s ten macroinvertebrate monitoring
sites. A discussion of the 2024 assessment can be found in Section 8.5.
8.7.3 Dissolved Oxygen
OAR 340-041-0016 specifically addresses dissolved oxygen (DO). Water bodies listed as
spawning areas must provide a minimum of 9.0 mg/L (or 95% saturation). Cold-water aquatic
habitat must have DO concentrations of at least 6 mg/L, a rolling 30-day floating average of the
daily mean concentration of 8 mg/L, or a minimum 6.5 mg/L when calculated as the minimum
7-day rolling average. Cool-water aquatic habitat must provide a minimum of 4.0 mg/L, a
minimum of 5.0 mg/L when calculated as the minimum 7-day rolling average, or a rolling 30-
day floating average of the daily mean concentration of 6.5 mg/L.
DO was listed for Fanno Creek and its tributaries (cool and cold-water classes), Tryon Creek
(Palantine Hill Creek to the Willamette River), and the Tualatin River (McFee Creek to the
Tualatin River). Streams potentially affected by this listing include Ball Creek, Carter Creek,
Nettle Creek, and streams on the south side of the City. Instream monitoring sites that could
provide information regarding this listing include Ball Creek bel Kruse Oaks, Carter Creek abv
Bangy, and Nettle Creek abv Tryon. Carter Creek and Ball Creek are included in the Tualatin
River TMDL for DO and a discussion of their WLAs is in in Section 8.6.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 48
DO levels for Nettle Creek ranged from 7.4 mg/L to 13.4 mg/L in FY2425. During the spawning
period (Oct 15 to May 15), the levels ranged from 8.4 mg/L to 13.4 mg/L. These concentrations
exceed the minimum DO criteria of 9.0 mg/L required by OAR 340-041-0016.
8.7.4 Temperature
Temperature is covered by a TMDL for the Willamette River and its tributaries including all of
the streams within the City’s jurisdiction. A detailed discussion of the temperature data at the
City’s instream monitoring sites is available in the annual report of the City’s TMDL
Implementation Plan (see Appendix B).
The Carter Creek watershed has the lowest canopy cover of the City, has an east-west
orientation, is primarily privately-owned, and has a major commercial/multi-family element,
e.g. large parking lots and buildings in the lower watershed where the sampling site is located.
In summary, five of the six monitoring sites met the temperature TMDL. While Carter Creek
continues to be above the temperature TMDL, it has decreased slightly from the maximum
temperature reached during the period starting July 1. 2017 and ending June 30, 2022.
8.7.5 Metals
Fanno Creek, in the Tualatin River basin, was listed in the 303(d) assessment for iron (total),
copper, chromium+6, and zinc. Copper was listed only for the segment from Carter Creek to the
Tualatin River. The Tualatin River from McFee Creek to the Willamette River was listed for iron
(total). Oswego Creek was listed for copper and zinc. All listings were for aquatic life criteria.
Streams potentially affected by the Fanno Creek impairments for iron, chromium, and zinc
include Carter Creek and Ball Creek whereas the copper listing potentially affects only Carter
Creek.
The listing for Oswego Creek stated that it was for all tributaries; therefore, as a conservative
analysis, the potential streams affected by the Oswego Creek listing are those that discharge to
Oswego Lake. Instream monitoring sites on Lost Dog Creek, Springbrook Creek, and Boones
Ferry at Bryant can provide information on copper and zinc.
The Tualatin River listing for total iron potentially affects Carter Creek, Ball Creek, and streams
on the south side of the City. The City started collecting total iron on a quarterly basis in FY2223
but does not have enough water-quality data to support an analysis of the 303(d) listing.
8.7.5.1 Copper
The City uses the Biotic Ligand Model (BLM) to determine the copper (Cu) threshold given
specific parameters such as anions, cations, hardness, and dissolved organic carbon (DOC)
which are collected quarterly at the City’s monitoring stations (hardness is collected monthly).
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 49
The model uses these inputs to determine the acute and chronic thresholds for copper and
assigns them a toxicity unit (TU). A TU greater than one (1) indicates a likely exceedance of the
threshold.
The median concentrations appeared to be well below the chronic and acute concentration
thresholds for Carter Creek and Lost Dog Creek, however the monitoring sites at Bryant and at
Springbrook appear to be elevated TUs (see Table 23). Interestingly, the copper concentrations
with TUs above 1 were estimated (below the detection limit of 2 µg/L) at Springbrook and at
the lower end of the discharges for the four events (August and November). The Boones Ferry
at Bryant site also had estimated copper concentrations (below the detection limit) when their
TUs were above 1. In general, the Boones Ferry at Bryant site has low flows except during storm
events; the area is also called “Lamont Springs”.
Table 23. Copper Concentrations for Selected Streams in Lake Oswego
Monitoring
Station
WQ-Limited
Stream
Median
Concentration
(µg/L)
Median
Acute
Threshold
(µg/L)
Median
Chronic
Threshold
(µg/L)
Acute
Toxicity
Unit Range
Chronic
Toxicity
Unit Range
Carter Creek abv
Bangy Fanno Creek 1.2 7.0 4.4 0.0 – 0.4 0.0 – 0.6
Boones Ferry@
Bryant Oswego Creek 1.7 2.2 1.4 0.2 – 1.3 0.3 – 2.2
Lost Dog Creek @
Lake Front Oswego Creek 1.5 7.5 4.6 0.1 – 0.3 0.1 – 0.5
Springbrook Creek
@ Iron Mtn Oswego Creek 0.9 1.5 1.0 0.5 – 1.0 0.8 – 1.6
8.7.5.2 Zinc
The acute and chronic thresholds for dissolved zinc are calculated via an equation from Table
30 in OAR 340-041-8033 and are based on hardness.
As shown in Table 24, the concentrations at all instream monitoring sites were below the
thresholds for acute and chronic concentrations of zinc.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 50
Table 24. Zinc Concentrations of Selected Streams in Lake Oswego.
Monitoring Site
Hardness,
mg/L
Maximum
Dissolved Zinc
(µg/L)
Acute
Threshold
(µg/L)
Hardness,
mg/L
Median
Dissolved Zn
(µg/L)
Chronic
Threshold
(µg/L)
Ball Creek 59.3 37.8 75.3 62.7 23.1 79.6
Carter Creek 57.0 23.5 72.8 96.6 7.9 113.8
Lost Dog 40.3 21.7 54.3 27.6 18.8 39.7
Springbrook Creek 48.7 22.6 63.8 54.4 4.9 70.8
Boones Ferry 57.6 43.0 73.6 45.8 17.0 60.6
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2425 Page 51
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Total Maximum Daily Load.
Oregon Department of Environmental Quality, 2021, NPDES MS4 Phase I Permit.
Oregon Department of Environmental Quality – Water Quality Division, 2022, Methodology for
Oregon’s 2022 Water Quality Report and List of Water Quality Limited Waters, Water Quality
Assessment Program.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A1
APPENDIX A SWMP IMPLEMENTATION STATUS, FY2425
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A2
Table A.1. SWMP Implementation Progress
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Program
Management
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Provide funding to properly
meet the NPDES MS4
requirements.
Track the funds used for the reporting
fiscal year and the expected fund
disbursement for the subsequent fiscal
year.
Since FY1920, the City has spent an average of $10.3
million annually on stormwater operations and
management, capital projects, parks facilities, and
natural resource management. In FY2425, the City
spent approximately $20.7 million dollars for
improvements in its parks and natural areas and
$5.2 million dollars on capital projects and
stormwater operations and maintenance (see Table
1). An additional $143,000 was spent on private
natural resources management.
Program
Management
Mercury
Chlorophyll α
DO
pH
Temperature
Provide funds to partners
who provide stormwater
education.
Track the amount of funds targeting
stormwater education that were
disbursed to external groups.
The Tryon Creek Watershed spends approximately
$1,500 per year of the $30K provided to them
through the City's Habitat Enhancement Program.
The City provided approximately $50K to the
Backyard Habitat program to work with private
property owners on native restoration of their
property.
The City provides funds to several regional
stormwater education groups. The Regional
Coalition of Clean Rivers and Streams receive $5K
that it splits with the Clean Rivers Coalition (a
statewide group). The City is part of the Clean Water
Partners which works with KPTV to provide
stormwater education; they received $2K in FY2425.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A3
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Construction
Site Runoff
Mercury
Bacteria
Chlorophyll α
DO
pH
Review and update, if
necessary, the ESC manual
and procedures for
inspection and plan review
at least once every 5 years.
Summarize changes to the wet weather
requirements, municipal code, and ESC
Manual
The ESC manual has been updated to add straw
wattles as a permanent standard detail due to
common use in tree protection zones to avoid root
damage per arborist input in FY2425
There were no changes to the wet weather
requirements. Changes to the municipal code were
adopted by the City Council in November 2024 with
an effective date in December 2024. Proposed
changes affect the enforcement actions, remove an
exemption for residential landscaping and clarify
what is required for an ESC Plan.
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
100% of City staff with
erosion control
responsibilities will be
certified professionals.
Track the number of City staff that are
certified ESC professionals.
There are two staff who are currently certified
professionals in erosion and sediment control plan
review and inspections.
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
100% of City contractors
associated with City-initiated
projects with a stormwater
component will have a
certified erosion control
professional on staff.
Track the number of City-initiated
projects with a certified ESC
Professional and whether the project is
completed or under construction
There were four City-initiated projects (Parks and
Engineering) with certified ESC professionals as part
of the project team in FY2425. The Lake Oswego
Recreation and Aquatics Center was completed in
FY2425. Rassekh Park, Lake Oswego Middle School,
and Lakeview Blvd. were under construction in
FY2425.
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Require 100% of developers
of subdivisions, business, or
commercial projects to
retain a certified ESC
professional to submit ESC
plans and provide
inspections.
Track the number of ESC Plans
submitted by a certified ESC
Professional for developers of
complicated projects.
Two projects had an ESC professional on staff. Three
were in the Oswego Lake watershed and one was in
the Tualatin River watershed.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A4
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
100% of instream projects
will apply for and receive
Department of State Lands
(DSL) jurisdictional decision.
and approval.
Track number of applications required
to submit a DSL application and the
affected watershed.
Eight projects were required to contact DSL/USACE
before receiving their ESC permit; two were in the
Oswego Lake watershed, three in Willamette
watershed, and three in Nettle Creek.
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
100% of construction sites
with steep slopes will
provide coconut coir matting
or hydroseeding for final
stabilization.
Track the number of construction sites
with slopes > 15% and the watersheds
affected.
Three were required to provide additional
protection because of steep slopes. The riparian
projects were located in Nettle and Spirit Creek.
Three projects with >15% slopes were located in
Mountain Park, located in the Tryon Creek
Watershed.
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
100% of all riparian or
riparian-adjacent projects
will provide coconut coir
matting or hydroseeding for
final stabilization.
Track the number of riparian or
riparian-adjacent projects required to
use coconut coir blankets and the
watersheds affected.
Two projects were completed in, or were adjacent
to, riparian areas and were required to provide
additional ESC protection using coconut coir
blankets. One was in the Nettle and the other in
Tryon Creek watershed.
Construction
Site Runoff
Mercury
Bacteria
Chlorophyll α
DO
pH
100% of construction sites
and professional landscaping
projects disturbing 500 sq ft
or more of soil will provide
erosion controls.
1. Track the number of ESC permits
issued.
2. Track the number of pre-construction
inspections (inspection code 6010) and
the number of final inspections
(inspection code 6050).
3. Track the number of additional
inspections for projects exceeding a
duration of 6 months.
There were 165 ESC permits issued in FY2425. There
were 192 inspections completed after the ESCs
were installed but work was started on the site.
Final inspections, after final stabilization has been
completed, were conducted for 178 projects. There
were 13 inspections conducted for projects that
were in construction longer than 6 months.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A5
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Construction
Site Runoff
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Maintain an escalating
enforcement program for
the ESC program.
1. Track the number of stop work
orders and violations issued; categorize
by watershed.
2. Track the number of projects with
more than 1 stop work order or
violation and the affected watershed.
One citation was issued for a project located in the
Nettle Creek watershed during FY2425. In addition,
four stop-work orders were issued, two located in
Nettle Creek and two in Lost Dog.
IDDE
Mercury
Bacteria
Chlorophyll α
DO
pH
Review and update, as
necessary, the IDDE manual
at least once per permit
term.
Summarize any changes to the IDDE
manual and the implementation date of
the changes.
The IDDE was reviewed and no changes were made
at this time.
IDDE
Mercury
Bacteria
Chlorophyll α
DO
pH
Review and update, as
necessary, the spill response
procedures every 2 years.
Summarize any changes to the Spill
Response SOP.
There were no changes in spill response policies or
procedures, however, the contact list was update due
to new personnel.
IDDE
Mercury
Bacteria
Chlorophyll α
DO
pH
Provide a complaint hotline
number.
Track the number of complaints
received through the hotline number.
According to Public Works staff, no spill-related
complaints were received from the complaint
hotline in FY2425.
IDDE Mercury
Reduce potential pollutants
of concern from entering
waterways.
Track the number of Drug TakeBack
events and the pounds of drugs that
were collected.
FY2425, staff held events that resulted in 407
pounds of drugs being removed from the solid
waste stream. On average, the City has removed an
average of 811 pounds of drugs from the solid
waste stream each year since FY2021.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A6
IDDE
Mercury
Bacteria
Chlorophyll α
DO
pH
Respond to 100% of valid
spill complaints within 24
hours if it is a threat to
human health or the
environment and within 2 to
4 working days of all others.
1. Track the number of spills responded
to by City staff.
2. Track the number of spills reported
to DEQ and their OERS numbers.
3. Track the number of events where
TVF&R was contacted for assistance.
4. Summarize resolutions and number
of citations issued
No citations were issued in FY2425 related to spill
incidents. A minor oil spill occurred in the
Springbrook watershed; the spill was contained and
fully cleaned. In addition, a reported drain spill at an
apartment complex was investigated after a delayed
report from a resident. No evidence of a spill was
found, and educational materials were provided to
the residents.
A car wash facility in the Oswego Lake watershed
received warning letters for ongoing illicit discharge.
The car wash discharge has since been resolved.
Lake Oswego High School was reported to DEQ for
dumping heated and treated swimming pool water
into a catch basin on their property. DEQ contacted
staff, they have since received guidance on the
proper disposal of swimming pool water as well as a
warning to the superintendent and all maintenance
staff, maintenance staff fulfilled additional
educational requirements made by staff.
The Lake Oswego Fire Department (LOFD)
coordinated with Public Works Maintenance and
Operations to clean fluids released from automobile
crashes. All fluids were properly contained and
cleaned within 24 hours of each incident.
The City had one SSO in FY2425 in the Springbrook
Creek watershed. While most SSOs tend to occur
because of tree roots or private lateral failure, this
incident was caused by the top of a tree impaling
the main line. Contractor was able to make timely
repairs.
The Tualatin Valley Fire & Rescue (TVF&R) and LOFD
have an agreement for consultation and incident
response for hazardous material spills. There were
no incidents requiring consultation or response by
TVF&R in FY2425.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A7
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
IDDE
Bacteria
Chlorophyll α
DO
pH
Reduce number of active
septic systems and illicit
connections.
1. Track number of properties that
decommissioned their septic system
and connected to the wastewater
system. Track the affected watershed.
2. Track the number of cross-
connections discovered and the actions
taken (warnings, enforcement,
abandonment, or re-connection). Track
the affected watershed.
There were no cross-connections discovered in
FY2425.
Staff inspect new wastewater connections and
monitor when septic systems are decommissioned.
Two septic systems were decommissioned in
FY2425. One was located in the Tualatin and the
other in the Oswego Lake watershed.
IDDE
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Using a rotational inspection
schedule, inspect each
outfall at least once during
permit term and inspect
each priority outfall at least
once every 2 years.
1. Track the number of outfalls and
priority outfalls inspected during dry
weather (72-hr antecedent dry period)
and the affected watersheds.
2. Track the number of inspected
outfalls and priority outfalls with
groundwater discharges.
3. Track the number of outfalls and
priority outfalls with a discharge that
was determined to not be
groundwater.
Staff were able to inspect all 17 priority outfalls
during FY2425. All outfalls were determined to be in
“good” condition.
Outfalls that had active flow or groundwater
discharge were tested with a handheld meter and
were determined to not have any indicator
parameters to indicate any issues with water
quality.
It was determined that 7 priority outfalls were due
to groundwater discharges.
Industrial /
Commercial
Program
Mercury
Chlorophyll α
DO
pH
Review and update, as
necessary, the strategy for
the Industrial and
Commercial Inspection
program once per permit
term.
Summarize changes to the strategy and
the implementation date.
There were no changes to the SOP adopted in late
2023.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A8
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Industrial /
Commercial
Program
Bacteria
Chlorophyll α
DO
pH
Reduce potential pollutants
of concern from entering
waterways.
Track the number of approved Land Use
applications, their case numbers, and
the affected watersheds for commercial
and industrial projects.
In FY2425, there were 18 applications for variances,
partitions, and lot line adjustments affecting 14
acres (see Table 4). The approved applications
resulted in an additional 6 lots created in the city.
Increased impervious area will result in all but seven
of the approvals. A majority of the cases were in the
Oswego Lake watershed and affected 8.2 acres.
Cases in Tryon Creek affected 3.9 acres, 1.59 acres
in Springbrook, and .23 acres in Willamette River
watershed.
Industrial/
Commercial
Program
Mercury
Chlorophyll α
DO
pH
Reduce potential pollutants of
concern from entering
waterways
1. Track the number of projects with
construction plan review and the
affected watersheds.
2. Track the number of projects completed
and the stormwater facilities
constructed.
Stormwater facilities were completed for four
commercial projects (1.41 acres), 1 public
improvement project (1 acre), and 48 SFR projects
(4.48 acres) resulting in 6.90 acres of created or
replaced impervious area (see Table 6). Most of the
development (SFR and non-SFR) was in the Oswego
Lake watershed at 3.44 acres.
During FY2425, 41.8% of stormwater treatment was
achieved through green infrastructure (GI), with
infiltration facilities handling 67.6%. Underground
Injection Controls (UICs) were used to treat
approximately 78.2% of the total impervious area.
One Institutional project was completed in FY2425
affecting 4.67 acres. The project was located in the
Oswego Lake watershed. To treat the 4.67 acres of
impervious, the stormwater facility used was a flow
through rain garden.
No industrial projects were completed in FY2425.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A9
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Industrial /
Commercial
Program
Mercury
Chlorophyll α
DO
pH
Reduce potential pollutants
of concern from entering
waterways.
Track the number of businesses
required to contact DEQ regarding their
industrial stormwater(1200-Z) program
and the affected watershed.
No businesses were required to contact DEQ in
FY2425.
Industrial /
Commercial
Program
Mercury
Chlorophyll α
DO
pH
Reduce potential pollutants
of concern from entering
waterways.
Track the number of completed
inspections, the facility categories, a
summary of the inspection results, and
the affected watershed.
During FY2425, staff completed nine commercial
inspections and worked closely with commercial
property owners to educate them and ensure
stormwater systems were properly functioning or
repaired as needed. Most management activities
involved catch basin cleaning and verifying that
each commercial facility maintained a current
operations and maintenance plan. The second most
common issue identified involved bioswales, which
often required regrading to their original design and
replanting to full vegetation coverage.
Pollution
Prevention
Chlorophyll α
DO
pH
Review and update, as
necessary, the Emergency
Operations Plan every 5
years.
Provide a summary of any changes in
the Emergency Operations Plan that
affect stormwater, e.g. Winter Events.
The City adopted its 3rd Emergency Operations Plan
(EOP) on June 4, 2024. The last version of the EOP
was adopted in 2017.
Pollution
Prevention
DO
pH
Control or prevent materials
from fire training activities
from entering the
stormwater system.
1. Review and update, if necessary, the
procedures for controlling or
preventing training materials from
entering the stormwater system.
Summarize any changes in procedure
and the implementation date.
2. Track the number of fire training
events within the City's jurisdiction and
their location.
There were no changes to the procedures used for
fire training in regard to stormwater. A one-hour
course, "NFPA 1500 Hazardous Material
Identification and Spill Prevention", was completed
by 46 LOFD staff.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A10
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Pollution
Prevention
Mercury
Bacteria
Chlorophyll α
DO
pH
Provide stormwater programs
that meet industry standards.
Review, update, and provide a summary
of any changes to the inspection and
maintenance procedures for City-
owned bridges at least once per permit
term.
Inspection procedures were updated in 2020. Next
revision due in 2026. In general, ODOT performs
bridge inspections and maintenance on behalf of
the City.
Pollution
Prevention
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Provide stormwater programs
that meet industry standards.
Review, update, and summarize any
changes to the inspection and
maintenance procedures for
conveyance structures and non-
vegetated stormwater treatment
facilities at least once per permit term.
There were no changes for FY2425. Metric is due in
2026.
Pollution
Prevention
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Maintain the stormwater
system inventory.
1. Track the number of structures, by
type, that are City-owned, e.g. catch
basins, manholes, detention tanks,
vaults, water-quality vaults, etc.
2. Track the number of feet of
stormline, ditches, and culverts that are
City-owned.
3. Track the number of vegetated
stormwater facilities that are City-
owned.
The City’s public stormwater system contains over
132 miles of stormline, 57 ditches, 300 culverts,
over 4,200 catch basins, and approximately 3,000
manholes.
138 vegetated facilities were inspected by Public
Works and Parks.
Pollution
Prevention
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Conduct a rotational
inspection schedule such
that 100% of the public catch
basins are inspected and
cleaned at least once during
the permit term.
Track the number of catch basins that
are inspected and cleaned each year. Staff inspected and cleaned 1, 1540 catch basins.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A11
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Pollution
Prevention
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Conduct a rotational
inspection and cleaning
schedule of the public
stormwater conveyance
system.
1. Track the number of feet of City-
owned stormline, ditches, and culverts
that are inspected and cleaned each
year.
2. Track the number of detention tanks,
vaults, water-quality vaults, and other
non-vegetated water quality structures
that are inspected and maintained each
year.
The City inspected and cleaned 1540 catch basins,
191 manholes, 4,008 feet of stormline, 12 UICS, 900
feet of ditches, 175 culverts (8* a year), four vaults
with eight filters replaced, and five ponds with 45
cubic yards or debris removed.
Pollution
Prevention
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Properly dispose 100% of the
debris from street sweeping.
Track the number of lane miles swept
and the disposal method for the
collected debris.
Staff swept 4,630 lane miles in FY2425. Leaf debris
was composted and the remaining debris was sent
to the landfill.
Pollution
Prevention
DO
pH
Balance road safety and
water quality during winter
storm events.
1. Track the number of gallons of
magnesium chloride and the number of
cubic yards of sand that are used
annually for winter events.
2. Track the number of 12-hr shifts
requiring winter event activities.
3. Track the number of lane miles
where magnesium chloride was used
(for in-truck GPS units).
Staff laid down 120 cu yards of sand during winter
events. Staff worked two 12-hr shifts in FY2425.
Staff dispersed 6,450 gallons of magnesium chloride
during winter events. The dispersal rate is 18 gallons
per lane mile and, while it translates to
approximately 360 lane miles, many streets receive
magnesium chloride treatment before each storm
event.
Pollution
Prevention
Mercury
Bacteria
Chlorophyll α
DO
pH
Properly dispose 100% of the
debris from asset cleaning.
1. Track the estimated tonnage of
debris that is sent to the landfill and
whether any debris is sent to a
hazardous waste landfill.
2. Track the estimated volume of debris
that is composted.
Street sweeping and maintenance of the
stormwater system resulted in 1,200 cu yards of
debris being removed from structures and streets.
1,291 tons of debris was sent to the landfill.
Almost 800 cu yd of leaf debris was collected for
composting in FY2425.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A12
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Pollution
Prevention
Mercury
Bacteria
Chlorophyll α
DO
pH
Reduce illicit discharges from
wastewater.
1. Track wastewater basins inspected as
part of the I/I program.
2. Track the watersheds where repairs
were needed and the funds required
for the repairs.
3. Track the funds spent on the total
program (inspections and repairs)
annually.
The City videoed approximately 96,126.314 feet of
wastewater lines in FY2425. Inspections and repairs
on manholes, cleanouts, and mainlines were
completed at 51 locations throughout the City at a
cost of $159K. A majority of the repairs were at
locations in the Oswego Lake watershed (36
locations), (12) locations in Willamette watershed
and (3) locations in Tualatin.
Pollution
Prevention
Mercury
Chlorophyll α
DO
pH
Provide stormwater
programs that meet current
industry standards.
Review, update, and summarize
changes to the Integrated Pest
Management plan at least once per
permit term.
The Parks Department reviews the IPM plan every
January and updates it as necessary. The IPM was
changed in FY2324 to include a pesticide report to
the Parks and Recreation Board.
Pollution
Prevention
Mercury
Chlorophyll α
DO
pH Monitor pesticide use.
Track the pesticides (by active
ingredient and quantity) used on City-
owned property.
The median amount of pesticides used between
FY1718 and FY2425 (7 years) is 120 gallons and 300
pounds. The pesticides used most often each year
have been glyphosate, triclopyr, and
pentachloronitrobenzene based on the gallons and
pounds used of the formulations and not the actual
amount of the active ingredient.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A13
Program
Management
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Provide stormwater
programs that meet current
industry standards.
1. Review and update the municipal
code to comply with the 2021 permit.
2. Track progress and adoption date of
revised code sections.
3. Provide summary of changes.
4. Track the number of public
comments received.
The municipal code review resulted in four change
categories. One was a language update; another
was a definitions update - merging when applicable
and either adding or deleting those that were
needed; a third category was consolidation of
requirements such as referring back to the code
chapter on stormwater management rather than
restating the same requirement in another chapter;
the fourth category were those changes that were
specific to the code chapters on stormwater
management and erosion control.
Code requirements were removed when they were
process-oriented (such as the steps for a
stormwater review), are the enforcement
responsibility other jurisdictions (such as DEQ's
1200-C permit), or were no longer required. Code
chapters for street design and stormwater design
were changed to reference all of the design
documents such as the Engineering Design
Standards. A variance procedure with approval
criteria was added to the code as well as changing
the enforcement procedures in the stormwater and
the erosion control chapters to ensure an escalating
enforcement procedure.
Workshops were held for the Planning Commission
and the City Council in June 2024. In August, an
online comment form was posted to the City's
website and a notice about the proposed code
amendments was sent to developers and engineers
who work within the City. One comment was
received and was incorporated into the proposed
code amendments. No comments were received at
the Planning Commission hearing in September
2024. No comments were received at the City
Council hearing in November 2024. The City Council
adopted the proposed code revisions with an
effective date in December 2024.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A14
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Program
Management
Mercury
Chlorophyll α
DO
Temperature Monitor impervious area.
Summarize municipal code changes that
affected sensitive lands, increased
density, or increased impervious area.
The following changes to the Community
Development Code were approved in July 2025:
clarified a building cannot cantilever into a 5-foot
landscape buffer along an access lane; revised the
Native Plant List to classify Snowberry & Douglas
spiraea as shrubs; and, added a Standard and
Enhanced Code Enforcement Fee for violations in a
Sensitive Lands District based upon square footage
of disturbed area. In addition, the city amended
code in keeping with the State-mandated Climate-
Friendly & Equitable Communities efforts to
minimize impervious area by removing minimum
parking requirements, adding maximum parking
requirements, and adopting new "large" surface
parking lot standards if 1/2 acre in size or greater.
Post-
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
Provide stormwater
programs that meet current
industry standards.
Review, update, and summarize
modifications to the engineering design
standards and standard drawings that
affect stormwater at least once per
permit term.
There were no changes to the engineering
standards or standard drawings in FY2425.
Post
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
Provide stormwater
programs that meet current
industry standards.
Review and update the SWMM by
December 1, 2024. Provide a summary
of the changes
The City is working on an update and expects to be
completed by January 2026. A summary will be
provided with the FY2526 report.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A15
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Post
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
Provide stormwater
programs that meet current
industry standards.
Review and update LID/GI prioritization
strategy by December 1, 2023. Provide
a summary of the changes.
The City submitted its LID/GI strategy at the end of
November, 2023. The strategy requires infiltration
of the 10-yr storm as the first tier which includes
green infrastructure. If onsite infiltration is not
possible due to soil properties at the site, a
developer can use Tier 2 facilities which provide
extended filtration (e.g. flow-through planters) prior
to discharging stormwater offsite. Developers can
only use the third tier of facilities if they can show
that using Tier 1 or Tier 2 facilities is not technically
feasible because of setbacks, topography, or soil
properties.
Post
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
Ensure stormwater
treatment for 95% of
construction projects
exceeding stormwater
thresholds.
1. Review and update, as necessary,
the technical feasibility criteria for
stormwater treatment. Summarize the
criteria and provide the implementation
date.
2. Track the number of projects that do
not provide stormwater treatment due
to technical infeasibility, the affected
watershed, and the reason for the
exception.
The City is updating its technical feasibility criteria in
concert with the stormwater management manual
(SWMM) update. The new criteria require onsite
retention of the 10-yr storm unless site soil
properties or groundwater tables do not permit
infiltration. The second tier of stormwater
treatment requires extended filtration through
vegetated facilities or media cartridges. The third
tier allows all other stormwater facilities allowed by
the SWMM. The design professional must provide
technical reasons for the inability to Tier 1 facilities
when proposing the use of Tier 2 facilities. The
design professional must also provide technical
reasons for the inability to use Tier 1 and Tier 2
facilities before proposing the use of a Tier 3 facility.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A16
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Post
Construction
Site Runoff
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Monitor changes in
impervious area.
When new LiDAR data becomes
available, complete an impervious
analysis. Report on the amount of
impervious area within the City and
within the UGB that was derived from
the updated LiDAR data.
New LiDAR data was collected in the summer of
2024. Preliminary data will be available in the spring
of 2025; however the City does not expect to have
full use of the data until 2026.
Post
Construction
Site Runoff
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Monitor changes in
impervious area.
1. Track changes in the UGB boundary
and the planned zoning for the changes.
2. Track the number and type of
approved changes in zoning and the
affected watershed
3. Track the number of approved land
use requests and the affected
watershed.
There were no changes in the UGB boundary or
zoning changes. There were eight land-use cases
involving 2-lot and 3-lot partitions that will increase
the impervious area in the City. Three land-use
cases involving lot line adjustments may result in
increased impervious area if they proceed to future
development.
Post
Construction
Site Runoff
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Monitor changes in
impervious are.
1. Track the number, acreage, and
affected watershed for annexed
properties.
The City has approximately 13 sq miles within its
urban service area and 11.5 sq miles within its
jurisdiction. Four properties (4 lots) were annexed
into the City in FY2425 (see Table 3) with a
connection to the public wastewater system for
future development being the most cited reason for
annexation.
Post
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
Ensure stormwater
treatment for 95% of
construction projects
exceeding stormwater
thresholds.
Track the number of land-use
applications reviewed, the number
required to manage stormwater, and
the affected watersheds.
Thirty-seven land-use applications exceeded the
stormwater thresholds and were reviewed in FY2425.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A17
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Post
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
Review 100% of construction
plans that require public
stormwater improvements.
1. Track the number of projects
requiring public stormwater
improvements
2. Track the status of public
improvements, e.g. in review, in
construction, in warranty period,
accepted by City.
These included the construction of a swale, a
drywell, and two infiltration planters. The drywell is
located in the Tualatin watershed, the swale in the
Willamette watershed, one planter in the Tryon
Creek watershed, and the other in the Oswego Lake
watershed. All projects were completed and are in
the City's 1-yr warranty period.
Post
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
Ensure stormwater
treatment for 95% of
construction projects
exceeding stormwater
thresholds.
Track the number of subdivisions that
were reviewed and the affected
watershed.
No subdivisions in the Oswego Lake watershed were
reviewed and approved for construction in FY2425.
Post
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
Ensure stormwater
treatment for 95% of
construction projects
exceeding stormwater
thresholds.
1. Track the number of construction
plans reviewed.
2. Track the number of construction
projects that re-developed 3,000 sq ft or
more of impervious area.
3. Track the number of stormwater
facilities, by type, that were approved
for construction.
In FY2425, staff reviewed 53 construction plans of
which 44 were classified as large projects.
Post
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
Reduce TSS 80% prior to
discharge offsite for 95% of
the projects that exceed
stormwater thresholds.
1. Track the number of projects with an
offsite discharge.
2. Track the stormwater facilities
constructed to treat stormwater before
discharge offsite.
There were 14 properties with facilities that
discharged stormwater offsite.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A18
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Post
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
Ensure stormwater
treatment for 95% of
construction projects
exceeding stormwater
thresholds.
Track the amount of impervious area
treated by constructed private
stormwater facilities; categorize the
stormwater facilities by watershed and
type, e.g. planter, infiltration trench,
etc.
Stormwater facilities were completed for four
commercial projects (1.41 acres), 1 public
improvement project (1 acre), and 48 SFR projects
(4.48 acres) resulting in 6.90 acres of created or
replaced impervious area (see Table 6). Most of the
development (SFR and non-SFR) was in the Oswego
Lake watershed at 3.44 acres.
Infiltration galleries and drywells accounted for the
largest portion of treated impervious area,
representing a combined 56.6% of the total (see
Table 7). Filtration planters and a water quality vault
treated an additional 28.8% of the area.
During FY2425, 41.8% of stormwater treatment was
achieved through green infrastructure (GI), with
infiltration facilities handling 67.6%. Underground
Injection Controls (UICs) were used to treat
approximately 78.2% of the total impervious area.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A19
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Post
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
Ensure stormwater
treatment for 95% of
construction projects
exceeding stormwater
thresholds.
Report the number of City-initiated
projects that included a stormwater
component. Include its phase (in design,
under construction, or completed) and
the affected watershed.
Eight CIP projects were completed in FY2425 that
had a stormwater component (see Table 8). Two
projects were located in the Oswego Lake, Tryon
and Springbrook watershed, 1 in Tualatin and the
pavement rehabilitation projects were distributed
across multiple watersheds.
FY2425 seven projects and watershed In design: 5th
St/Lake Bay-Oswego Lake, Redfern Ave- Oswego
Lake, UIC Retrofit #3- Tualatin, ADA Curb Ramps-
Various, Glenwood Ct- Willamette, Pilkington
Pathway- Oswego Lake, Carman Pathway Tualatin &
Oswego Lake.
FY2425 four projects under construction and
watershed: Tree Top / Meadowlark-Oswego Lake,
Lakeview Blvd-Oswego Lake, Atwater Outfall- Tryon
Cree, and 2025 Pavement Rehab -Various.
Post
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
Ensure stormwater
treatment for 95% of
construction projects
exceeding stormwater
thresholds.
Track the number of stormwater
facilities constructed, the type (planter,
infiltration trench, etc.), impervious
area treated, and the affected
watersheds for new public stormwater
facilities.
The LORAC project constructed 4 new stormwater
facilities (2 on the City-owned site and 2 in the
public ROW on Stafford Rd) to treat a total of
227,535 sq. ft. (5.22 acres) of impervious area. The
Golf Course project does not include any impervious
area, and it shares the large stormwater pond with
the LORAC project. Three vegetated stormwater
ponds were constructed at Rassekh Park in Phase 1
to treat 48,118 sq. ft. (1.10 acres) of impervious
area. All projects are located in the Tualatin River
Watershed. All the facilities are vegetated LIDA
facilities.
Four projects were completed in FY2425 to provide
public improvements. These included the
construction of a swale, a drywell, and two
infiltration planters. The drywell is located in the
Tualatin watershed, the swale in the Willamette
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A20
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
watershed, one planter in the Tryon Creek
watershed, and the other in the Oswego Lake
watershed. All projects were completed and are in
the City's 1-yr warranty period. ditch and two
swales (total of 107 ft treating 2,548 sq ft of
previously untreated street) in the Tryon Creek
watershed.
Post
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
Require CCTV of all new pipe
systems at substantial
completion of the project to
ensure proper connections.
Track the amount of new pipe inspected
(in linear feet) and categorize as either
part of a public improvement or a
capital improvement project.
In FY2425, the City required video of 4,400 feet of
stormline.
Post
Construction
Site Runoff
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Review and update, as
necessary, the procedures
and criteria for inspecting
stormwater facilities at least
once per permit term.
1. Summarize changes to current
procedures and criteria for inspected
stormwater facilities.
2. Track the implementation
month/year.
There were no changes to the procedures and
criteria for inspection of stormwater facilities.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A21
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Post
Construction
Site Runoff
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Annually inspect 90% of the
public stormwater vegetated
facilities.
Track the number and type of each
public vegetated stormwater facility
that was inspected, the affected
watershed, and if repair or restoration
was required.
In FY2425, four water-quality vaults were inspected
and cleaned with eight cartridges replaced in the
vaults. City staff and contractors maintain
approximately 140 vegetated stormwater facilities
located throughout the city with a majority of the
facilities located in the Oswego Lake watershed (see
Table 11). In FY2425, staff and contractors replaced
dead trees in planters along Boones Ferry Rd, dead
plants and trees in planters along D Avenue and
Kerr Pkwy.
Post
Construction
Site Runoff
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Inspect at least 50% of the
private stormwater facilities
constructed during the
permit term.
1. Track the number of properties that
were inspected; categorized by
construction year.
2. Track the impervious area treated by
the inspected facilities by watershed.
In FY2425, staff inspected 10 private (commercial)
stormwater facilities at approximately 2 acres of
impervious area. The following were the
construction years: 2022,2020,2018/2023, 2022,
2022, 2019, 2020, 2020, 2005, and 1996
In FY2425, staff inspected 11 private (residential)
stormwater facilities at approximately 0.20 acres of
impervious area. The following were the
construction years: 2018, 2021, 2020, 2022, 2018,
2024, 2014, 2015, 2015, 2014, 2014
Post
Construction
Site Runoff
Mercury
Chlorophyll α
DO
pH
Temperature
Provide a report to the
property owner outlining
deficiencies within 90 days
of the inspection and require
repairs or corrections.
1. Track the number of properties that
required follow -up.
2. Track the number of properties that
required enforcement and whether a
citation was issued.
3. Track the number of inspections that
required no follow-up.
No enforcement was necessary in FY2425. Follow up
to ensure businesses cleaned out catch basins or
residents to clean out UICs were approximately 10.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A22
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Retrofit/
Hydromodific
ation
Mercury
Chlorophyll α
DO
pH
Temperature
Reduce hydromodification
through City-initiated
projects.
Track the number and location of CIP
and Parks projects that provide a
reduction in stormwater volume or
velocity, e.g. infiltration facilities (see
Hydromodification Assessment 2015).
Five completed projects reduced hydromodification
in FY2425. In addition, six engineering projects
currently under construction or in design are
expected to further reduce hydromodification.
These projects are located in the Oswego Lake
watershed (two projects), the Springbrook Creek
watershed, and the Tualatin River watershed (see
Table 8).
Retrofit/
Hydromodific
ation
Mercury
Chlorophyll α
DO
pH
Temperature
Provide velocity reduction
for 10% of existing ditches
and 75% of newly
constructed conveyance
ditches through longitudinal
slope reduction and
broadening of side slopes
(whenever feasible) with
percentages based on the
perm it term.
1. Track number of linear feet of
retrofit conveyance ditches.
2. Track the linear feet of new
conveyance ditches constructed using
the velocity reduction criteria.
There were no retrofits or new ditches constructed
in FY2425.
Retrofit/
Hydromodific
ation
Mercury
Chlorophyll α
DO
pH
Temperature
Provide velocity reduction
for 10% of existing major
outfalls and 75% of newly
constructed outfalls
(whenever feasible) with
percentages based on the
permit term.
1. Track number of outfalls retrofitted.
2. Track number of new outfalls
constructed with erosion protection. There were no outfall retrofits in FY2425.
Retrofit/
Hydromodific
ation
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Retrofit public catchbasins
whenever feasible.
1. Track number of catch basins
converted for all capital improvement
projects.
2. Track the number of new and/or
sumped and snouted catchbasins.
The LORAC project constructed 26 new catch basins
and area drains, all of which are sumped and
trapped. There are no new or retrofit catchbasins as
part of the Golf Course project. Rassekh Park Phase
1 constructed 12 new catch basins/area drains, all of
which are sumped and trapped, and Phase 2 will
add 10 additional area drains.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A23
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Public
Education
Mercury
Chlorophyll α
DO
pH
Temperature
Educate professionals and
the general public about City
projects.
1. Track the number of CIPs with a
stormwater component that included
public comment.
2. Track the type of outreach avenues
such as newsletters, meetings, and
social media types
No CIP projects needed a public comment
component for FY2425.
Staff reviewed the municipal code to determine if
any changes were needed to comply with the
permit. Most of the changes were administrative in
nature and, on the direction of the City Council,
staff reached out to engineers and developers
working in the city to provide comments. Staff
received one comment. The general public was able
to provide comments at the Planning Commission
and City Council hearings held in October 2024 and
November 2024, respectively. No comments were
received from the public.
h d d b h
Public
Education
Mercury
Chlorophyll α
DO
pH
Educate businesses about
stormwater.
1. Track the type and number of
messages sent to businesses.
All non-SFR properties were sent a letter by mail
requesting type of business activity information via
a survey and the request is also posted on the City’s
website to help prioritize industrial facility
inspections. Once on the site they can access
educational information on business related
activities such as carpet cleaning, car washing,
signing up to be an ecofriendly business etc.
Planning also sends out relevant educational
information to new businesses that submit for a
business license.
We also post on social media and articles offering
educational content such as rain down the drain
only, leaf pick up, and how to get involved with
volunteering.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A24
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Public
Education
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Work with partners who
provide stormwater
education.
1. Track the message subjects of funded
groups and outreach types
2. Track the number of funded HEP
projects that include stormwater
education
The City partners with several regional stormwater
education groups to provide consistent messaging.
The RCCRS has provided stormwater messaging on a
variety of topics to residents of the tri-county
Portland metropolitan area (estimated population
of 1.8 million) for over 25 years. The CRC has over
60 partners across Oregon and Washington with a
mission is to build a bridge between clean water
and healthy communities and provide consistent
messaging across Oregon and southwest
Washington. The Clean Water Partners work with a
local TV station, KPTV, to provide public service
announcements (PSAs).
RCCRS messages included information on pet waste
disposal, pesticide/herbicide use, automotive fluid
disposal, and separate versus combined stormwater
systems. Key messages for the CRC include pesticide
and herbicide use. Future programs include
building on the lawn care series with information on
lawn alternatives such as eco-lawns, gardens, and
meadowscaping. In FY2425, PSAs by the Clean
Water Partners included messages on water
conservation, herbicide and fertilizer use, car
washing, good practices for fall maintenance, and
keeping leaves out of the public stormwater system.
Information regarding leaf disposal and keeping
catchbasins free of leaves were run as part of the
campaign throughout the wet weather season.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A25
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Public
Education
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Work with partners who
provide stormwater
education
Track the number of funded HEP
projects that include stormwater
education (or track the number of
volunteer hours or events)
Tryon Creek Watershed projects for FY2425:
• Oswegan Apts. Invasive removal, 1750 plants
installed.
• Timberline Tributary, Invasive removal, 1000
plants installed.
• Nettle Creek Invasive removal.
• Lake Oswego United Church of Christ, Invasive
removal
Oswego Lake Watershed Council Projects for
FY2425:
• Mountain park HOA Springbrook Creek watershed
• Woodside circle HOA common property Oswego
Lake Canal
• Village on the lake HOA
Public
Participation
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Review the stormwater
webpages annually.
Summarize changes made to the
website.
In 2025, the City made a significant effort to boost
public education and engagement. The stormwater
pages for the city were all updated throughout
2025, including an all new Education and Resources
Page. The city also developed a social media
template and cohesive branding for its stormwater
program to create a recognizable identity and boost
engagement and awareness.
Public
Participation
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Maintain general email
contact and USPS address for
stormwater questions.
Track number and subject matter of
comments received. No comments were received in FY2425.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A26
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Public
Participation
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
pH
Educate stakeholders about
the City's stormwater efforts
by posting the SWMP,
SWMM, and annual reports
on the website.
Track number and subject matter of
comments received.
The annual reports and adjunct documents were
posted to the City's website in FY2425 and staff
created an online comment form for people to
comment on them. No comments were received
regarding the annual reports or adjunct documents
in FY2425.
Public
Participation
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Provide 30 days of public
comment for the municipal
code changes required by
the MS4 permit prior to
submittal to DEQ.
Track number and subject matter of
comments received.
Staff provided workshops to the Planning
Commission and to the City Council in June 2024 as
part of the municipal code update. On the Council's
recommendation during the June workshop, staff
sent code amendment notification and request for
comment on August 20th to engineers and
developers that work in the City. One comment was
received regarding a definition and the requested
change was incorporated into the proposed
stormwater amendments. Public hearings were held
for the Planning Commission on September 23rd
and for the City Council on November 5th. The
Planning Commission and City Council hearings
require noticing to the public about the agenda
items and allow public comment. No public
comments were received for the Planning
Commission hearing or the City Council hearing.
Public
Participation
Mercury
Bacteria
Chlorophyll α
DO
pH
Provide 30 days of public
comment for the revised
SWMP prior to submittal of
permit renewal package.
Track number and subject matter of
comments received. This metric is due in 2026.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A27
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Public
Participation
Mercury
Chlorophyll α
DO
pH
Provide 30 days of public
comment for the Industrial
and Commercial Program
Strategy prior to submittal to
DEQ.
Track number and subject matter of
comments received.
All non-SFR properties were sent a letter asking for
comment on the program after the strategy was
placed on the City’s website with an online
comment form. Outreach consisted of articles in the
City’s two newsletters and multiple postings on the
City’s social media platforms. The City also reached
out multiple times to the Chamber of Commerce for
a question & answer session on the program but
was unable to schedule it.
Six comments were received; two comments were
about exemptions from the program, one was about
the inspection process, and three were about the
changes to the municipal code.
Public
Participation
Mercury
Chlorophyll α
DO
pH
Temperature
Provide 30 days of public
comment for the LID/GI
prioritization strategy prior
to submission to DEQ.
Track number and subject matter of
comments received.
Seven comments were received during the 30-day
comment period for the City's LID/GI strategy.
Three of the comments were related to tree canopy
and its benefits for stormwater, one was related to
limiting impervious area for tax lots, one was
related to artificial turf, and the last one was related
to maintenance of existing green infrastructure.
Staff are updating the stormwater management
manual and are incorporating these comments
when feasible.
Public
Participation
Mercury
Chlorophyll α
DO
pH
Temperature
Work with neighborhood
groups, i.e. Friends Groups,
to restore natural habitat. Track the number of volunteer events.
The City works with volunteer groups through its
Friends of Parks program and the Habitat
Enhancement Program. In FY2425, the City
sponsored 9 volunteer work parties across the city
to plant approximately 3000 native plants and
remove invasive plants.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A28
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Public
Participation
Chlorophyll α
DO
pH
Temperature
Provide stormwater
education to the public.
1. Track the number of messages and
whether any drop-off bins for leaf
collection were provided by the City.
The City provides yard debris bins to reduce the
amount of vegetation that gets tossed into nearby
streams or in the public stormwater system. Yard
debris bins for leaf collection were provided by the
City to 1 neighborhood in FY2425.
Training
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Provide training to
Employees with stormwater
responsibilities. Inform or
train staff when changes in
stormwater policy and
procedures are
implemented.
1. Track the number of current
employees with stormwater
responsibilities.
2. Track the number of employees with
stormwater responsibilities that
received training in the reporting year.
3. Track the number of employees with
stormwater responsibilities that have
been trained in policy changes and
procedures.
There are 65 staff in the City who are involved with
stormwater report metrics. Within the Engineering
program, there are 10 staff with stormwater
responsibilities. Two received training in FY2425.
Within the Public Works Maintenance department,
there are 7 staff with stormwater responsibilities,
and 15 that received spill response training. In the
Planning Department, 5 staff have stormwater
responsibilities but did not receive training in
FY2425. In the LOFD, 45 members that have
stormwater responsibility and 46 participated in
hazardous material identification and spill response
training. There are 2 staff from Parks that have
stormwater responsibilities; they did not receive
training in FY2425.
Training
Mercury
Bacteria
Chlorophyll α
DO
pH
Temperature
Provide training to
Employees that are new to
stormwater within 60 days
of assignation.
Track the number of staff new to
stormwater that received training. There were no new LOFD staff in FY2425.
Training
Mercury
Bacteria
Chlorophyll α
DO
pH
Provide stormwater
programs that meet current
industry standards.
Track the number of staff trained in
spill response.
Training on spill response was provided to 46 LOFD
staff.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page A29
Minimum
Measure
Affected
TMDL Goal Tracking Measure Progress
Training
Mercury
Chlorophyll α
DO
pH
Provide certified training to
Employees with commercial
industrial inspection
responsibilities.
Track the number of certified staff or
contracted staff.
Two staff members have inspection experience but
neither have certifications.
Training
Mercury
DO
pH
Provide training for
Employees applying
pesticides.
Track the number of staff and
contractors trained
The City employs 17 staff and 7 contractors who are
certified pesticide applicators.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page Bi
APPENDIX B TMDL IMPLEMENTATION PLAN ANNUAL REPORT
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page Bii
City of Lake Oswego, Oregon
TMDL IMPLEMENTATION PLAN
ANNUAL REPORT COVERING
JULY 1, 2024 TO JUNE 30, 2025
Prepared for the Oregon Department of Environmental Quality
December 1, 2025
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page Biii
ACRONYMS
CWA Clean Water Act
CWR Cold Water Refugia
DEQ Oregon Department of Environmental Quality
DMA Designated Management Agency
HEP Habitat Enhancement Program
LA Load Allocation
M-IBI Macroinvertebrate Index of Biological Integrity
MS4 Municipal Separate Storm Sewer System
NPDES National Pollutant Discharge Elimination System
PLRE Pollutant Load Reduction Evaluation
PREDATOR Predictive Assessment Tool of Oregon
SWMP Stormwater Management Plan
USEPA US Environmental Protection Agency
USGS US Geological Survey
WQMP Water Quality Management Plan
WLA Wasteload Allocation
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B1
B.1 INTRODUCTION
The Clean Water Act (CWA) was enacted in 1972 to “restore and maintain the chemical, physical,
and biological integrity of the Nation’s waters”. After point sources such as industrial facilities
and wastewater treatment plants were regulated as part of the National Pollutant Discharge
Elimination System (NPDES) program, the USEPA found that the nation’s rivers and streams were
still not providing sufficient water quality to meet their assigned beneficial uses. As a result, the
NPDES Municipal Separate Storm Sewer System (MS4) Phase I permit program was implemented
in 1990 to regulate stormwater discharges from municipalities, counties, and other entities that
met the MS4 permit thresholds of a population of 100,000 people or in an urbanized area. The
City of Lake Oswego (City) was issued a Phase I permit, known as the Clackamas Permit, in 1995.
Under Section 303(d) of the CWA, the Oregon Department of Environmental Quality (DEQ) must
identify watersheds that do not have sufficient water quality to meet their strictest beneficial
use. DEQ determines the total maximum daily load (TMDL) that each watershed can carry for all
parameters that are keeping the watershed from achieving its beneficial uses. Entities that have
discharges into a water-quality limited waterway are designated as management agencies
(DMAs) for these parameters. The City was determined to be a DMA for several parameters in
the watersheds under its jurisdiction (See Table B.1).
Table B.1 Watersheds affected by TMDLs of which the City is a Designated Management Agency
TMDL Basin
Issue
Date Parameter1
Tualatin River Basin
2001
Added:
• Temperature (surrogate – effective shade)
• Bacteria
1988/
2012
Updated 1988 Parameters:
• pH
• Chlorophyll a (surrogate – total phosphorous)
• DO (surrogates – TSS and VSS)
Lower Willamette River
including the Tualatin River
Basin
2006
and
2012
• Bacteria
• Mercury
• Temperature (surrogate – effective shade)
• pH
• Chlorophyll a (surrogate – total phosphorous)
• DO (surrogates – TSS and VSS)
Lower Willamette River
including the Tualatin Basin 2019 Updated:
• Mercury (surrogate – TSS)
1 – All parameters except temperature have wasteload allocations and are covered under the City’s NPDES MS4 Phase I permit;
TSS=total suspended solids; VSS=total volatile suspended solids)
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B2
B.2. TMDL IMPLEMENTATION PLAN REVISIONS
The Willamette River and Tualatin River have Water Quality Management Plans (WQMPs)
associated with their TMDLs. These WQMPs provide guidance for reducing the parameters that
have TMDLs and require a revision of the City’s TMDL Implementation Plan (“TMDL Plan”) every
five years.
In FY1819, the City revised its Plan in response to DEQ’s realignment of the Willamette River and
Tualatin River WQMP timelines. The City submitted its revised TMDL Implementation Plan to DEQ
in October 2018 for their review and received approval in May 2020. In response to the revised
mercury TMDL issued in 2019, the City submitted a mercury minimization plan in August 2022
and received approval of the plan in February 2023. The City revised its TMDL Plan in 2023 as
part of the regular 5-yr review cycle, submitted it to DEQ in December 2023, and received
approval in July 2024.
B.3 PROGRAM MANAGEMENT
The City uses funds from the Planning Department and the Parks and Recreation (Parks)
Department (Parks) to fund many of the activities required in its Plan. Two programs, the Habitat
Enhancement Program (HEP) and the Backyard Habitat Program, are directly funded by the City
in support of habitat restoration and preservation on private property. In FY2425, the City
disbursed approximately $60,000 through the HEP to local watershed councils. The City
disbursed $50,000 through the Backyard Habitat Program to assist property owners with private
riparian restoration.
In addition to providing funds for private property restoration, the Parks Department maintains
a robust public property restoration program through invasive removal and habitat
enhancement. The City disbursed approximately $323K for restoration of City-owned property.
The City provides access to its TMDL Plan and to annual reports on its website. The City received
no comments on these documents in FY2324.
B.4 TMDLS & STORMWATER MANAGEMENT
All of the constituents identified as needing a TMDL, except stream temperature, are regulated
through the City’s Phase I NPDES MS4 permit and have waste load allocations (WLAs). The City’s
stormwater management plan (SWMP) contains actions the City takes to meets its regulatory
obligations under its MS4 permit. The following actions in the SWMP address the TMDL
parameters assigned to the City (See Table B.2):
• Implementation of an erosion control program to reduce bacteria, phosphorous, and
mercury while increasing dissolved oxygen and stabilizing pH and temperature.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B3
• Implementation of a development review program to reduce stormwater volume and
streambank erosion thereby reducing bacteria, phosphorus, and mercury while
increasing dissolved oxygen and stabilizing pH and stream temperatures.
• Maintenance of the public stormwater conveyance and water-quality structures to
reduce bacteria, phosphorus, and mercury while increasing dissolved oxygen and
stabilizing pH and stream temperature.
• Implementation of a cross-connection program to reduce bacteria and phosphorous.
• Implementation of an illicit discharge program to reduce bacteria, phosphorus, and
mercury while increasing dissolved oxygen and stabilizing pH.
• Implementation of an inspection program for industrial and commercial properties to
reduce bacteria, phosphorous, and mercury while increasing dissolved oxygen and
stabilizing pH.
• Implementing a public education and public involvement program to reduce bacteria,
phosphorous, and mercury while increasing dissolved oxygen and stabilizing pH and
stream temperatures.
Additional information regarding these programs is provided in the City’s annual NPDES MS4
report.
Table B.2. TMDL Parameters and City Activities related to the NPDES MS4 Permit
Minimum Measure
(MS4 Permit)
TMDL Parameter (and Surrogate)
Bacteria
Dissolved
Oxygen
(TSS/VSS1) pH
Chlorophyll
α
(Phosphorus) Mercury
Temperature
(Effective
Shade)
Construction Site Runoff √ √ √ √ √ √
Post-Construction Site
Runoff √ √ √ √ √ √
Pollution Prevention in
Municipal Operations2 √ √ √ √ √ √
Illicit Discharge Program3 √ √ √ √ √
Industrial and
Commercial Program √ √ √ √ √
Public Education
and Public Participation √ √ √ √ √ √
1 TSS = Total Suspended Solids, VSS=Volatile Suspended Solids; 2 – includes street sweeping and pesticide reduction; 3 – includes
cross-connections
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B4
B.5 WASTELOAD ALLOCATIONS
Historical progress towards attaining the WLAs is found in the City’s 2016 Pollution Load
Reduction Evaluation (PLRE) and 2016 WLA Assessment. An analysis of the City’s progress
towards meeting its WLAs and a discussion regarding constituents listed in the 303(d) list are
provided each year in the City’s NPDES MS4 report. The following analysis is a summary of the
FY2425 water-quality data from the annual MS4 report.
B.5.1 Bacteria
Bacterial TMDLs have been set for the Tualatin River (all tributaries and ephemeral streams),
Springbrook Creek and the Willamette River (all tributaries).
Monitoring sites affected by the Tualatin River bacteria TMDL are Carter Creek and Ball Creek
(see Table 15). Geomean concentrations for Ball Creek and Carter Creek were below the 406
col/100 mL threshold for the winter and summer, Ball creek’s winter and summer geomean fell
below the threshold while Carter Creek was slightly above for the winter geomean but fell under
the geomean for summer.
Based on data collected by the City from 1997 to 2003, DEQ determined that bacterial
concentrations in Springbrook Creek must be reduced to 80% of their 1997 to 2003
concentrations (see Table 16). The bacterial data indicates that E. coli levels have decreased
compared to FY2324. Two months of data from FY2324 were missing, and one month recorded
levels at or above 2,420/mL due to an ongoing SSO event, there was also another SSO in the
Springbrook watershed in FY2425. Although a reduction was observed, the targeted geometric
mean threshold was not achieved for FY2425.
The Willamette River TMDL for bacteria requires a 78% reduction of the E. Coli concentrations
from 1997 to 2002. As shown in Table 17, the E. Coli geomean concentration show reductions of
14% for Nettle Creek, 70% for Lost Dog, and 42% for Boones Ferry. Reductions in the maximum
concentrations were 62% for Nettle Creek, 86% at Lost Dog, and 76% at Boones Ferry.
B.5.2 Dissolved Oxygen
The 2001 dissolved oxygen (DO) WLA for Fanno Creek and its tributaries required a minimum
concentration of 6.5 mg/L and a 50% decrease from pre-1998 concentrations for total suspended
solids (TSS) and total volatile suspended solids (VSS) from May 1st to October 31st.
Ball Creek and Carter Creek, as tributaries of Fanno Creek, are covered under the Fanno Creek’s
WLA. The minimum concentration of 6.5 mg/L for DO was met for Carter Creek but not for Ball
Creek in FY2425(see Table 18). A measure of the longer-term DO concentration experienced by
macroinvertebrates is the median DO concentration. The threshold concentration of 6.5 mg/L for
the median DO concentration was not met in both Ball Creek and Carter Creek for FY2425,
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B5
however there was an overall reduction for both minimum and median from both the baseline
and historical time periods.
Ball Creek saw a 94% reduction in the maximum TSS concentration and a 50% reduction in the
median TSS concentration meeting its reduction targets. The maximum and median VSS
concentrations also met the reduction target of 50%.
Carter Creek saw an 90% reduction in TSS from the baseline concentrations and a 47% reduction
in median concentrations not meeting the reduction target of 50% for median. The reduction for
VSS concentrations at Carter Creek was 94% for maximum concentrations and 94% for median
concentrations.
B.5.3 Chlorophyll α and pH
Chlorophyll α and pH TMDLs were set in the 2001 Tualatin River TMDL and revisited, in 2012, for
Oswego Lake and its tributaries. Total phosphorous is used as a surrogate measure for pH and
chlorophyll α. Water-quality data on total phosphorous and pH covered by this TMDL include
stream samples taken from the instream monitoring sites at Ball Creek, Carter Creek, Springbrook
Creek, Lost Dog Creek, and Boones Ferry at Bryant.
The maximum summer phosphorous concentration for the Tualatin River watershed was
determined by DEQ to be 0.46 mg/L with a median of 0.13 mg/L. Ball Creek met the pH range
and was below the median and maximum phosphorous WLAs. Ball Creek has seen a 57%
reduction in the maximum summer phosphorous concentration from historical levels.
Carter Creek met the pH range and was below the maximum phosphorous WLA but slightly above
the median phosphorous WLAs. Carter Creek has seen a 64% reduction from historical
concentrations for the maximum summer phosphorous concentration.
The maximum summer concentration for total phosphorous in the Oswego Lake watershed was
set by DEQ in 2001 to 0.09 mg/L with a maximum winter concentration of 0.15 mg/L (see Table
21). Monitoring sites within the Oswego Lake watershed include Springbrook Creek, Lost Dog
Creek, and the Boones Ferry at Bryant location .
B.5.4 Biocriteria
The threshold for biocriteria is “a sufficient (water) quality to support aquatic species without
detrimental changes in the resident biological community” (DEQ, 2022). Water bodies that have
biocriteria impairment include Fanno Creek and Tryon Creek. Tributaries to these streams that
are within the City’s jurisdiction include Ball Creek, Carter Creek, and Nettle Creek.
The City partnered with Water Environment Services, a co-permittee, to complete a
macroinvertebrate assessment in the Fall of 2021. Results were analyzed using the multi-metric
macroinvertebrate-based index of biological integrity (M-IBI) and the Predictive Assessment Tool
for Oregon (PREDATOR).
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B6
All of the sites were severely impaired under the PREDATOR assessment. Ball Creek showed a
slight decline in aquatic habitat quality; however, Oswego Creek and Nettle Creek showed a
steady increase in aquatic habitat quality. Springbrook abv Boones Way showed improvement in
habitat quality since 2013 but appears to be in stasis at the moment. The M-IBI method indicated
that seven of the ten sites were severely impaired with the remaining sites showing moderate
impairment. Oswego Creek, Ball Creek, and Carter Creek all showed slight improvements since
2013. Tryon Creek and Springbrook abv Boones Way showed a progressive decline since 2013.
Temperatures above 18.4°C indicate a likely shift in the composition of the macroinvertebrate
community from cool to warm water taxa. Seven of the ten sites assessed in 2021 were
experiencing stress from temperatures.
Fine sediment in streambeds in excess of 19% indicates a likely shift in the composition of the
macroinvertebrate community from pollution-sensitive taxa to taxa that are more tolerant of
degraded conditions. Six of the ten sites indicate stress from fine sediment, however three of
those sites experienced less sediment-induced stress since 2013. One site (Lost Dog at Lake Front)
dipped below the threshold for sediment-induced stress for the first time since 2013. None of
the sites showed an increasing trend in stress from fine sediment.
B.6 TEMPERATURE
Stream temperatures are affected by the amount of summer baseflow, stream channel widening
due to erosion caused by short-term high-volume runoff, stormwater temperature, removal of
riparian vegetation, and reduced diversity of riparian canopy. Of these causes, stormwater
temperature is the least harmful because the maritime climate in the Willamette Valley defines
our mild rainy winters and hot dry summers which result in very few rain events when air and
stream temperatures are at their highest.
B.6.1 Temperature TMDLs
Temperature maximums are set based on whether the stream is used for salmonid rearing,
migration, or spawning. The monitored sites within the City’s jurisdiction are cool-water habitat
which requires a maximum temperature of 17.8°C except for Nettle Creek, a tributary of Tryon
Creek, which has a maximum temperature of 13.0°C in the spawning season.
As shown in Table B.3, five of the six monitoring sites met the temperature TMDL. While Carter
Creek continues to be above the temperature TMDL, it has decreased slightly from the maximum
temperature reached during the period starting July 1. 2017 and ending June 30, 2022.
B.6.2 Temperature Mitigation
The City addresses temperature loading through the following metrics:
A. Preservation of existing riparian habitat through code enforcement of the sensitive lands
Resource Protection overlay.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B7
B. Enhancement of existing riparian habitat by funding restoration of public and private
properties and protecting or enhancing areas of cold-water refuge.
C. Monitoring tree canopy and effective shade through an analysis of LiDAR data.
D. Enhancement and protection of existing shade in riparian corridors through enforcement
of the City’s municipal code and by educating the public on the value of shade and trees.
E. Enhancement of cold water refugia.
Table B.3. Stream Temperatures at Selected Sites in Lake Oswego, FY2425 and FY1718-FY2122
Stream
Target Temperature Summer Maximum,
°C FY2425 FY1718-FY2122
Ball Creek 17.8 15.9 17.1
Carter Creek 17.8 19.3 20.6
Boones Ferry site 17.8 15.5 16.3
Lost Dog Creek 17.8 17.5 18.2
Springbrook Creek 17.8 14.5 16.0
Nettle Creek
13.0 (Oct 15-May 15)
17.8 (May 16-Oct 14)
10.9
16.0
13.0
17.0
B.6.2.1 Preservation of Riparian Habitat
The City’s Sensitive Lands Overlay affects approximately 120 acres and encompasses most of the
riparian corridors within its jurisdiction. In FY2425, there were 18 applications for variances,
partitions, and lot line adjustments affecting 14 acres (see Table 4). The approved applications
resulted in an additional 6 lots created in the city. Increased impervious area will result in all but
seven of the approvals. A majority of the cases were in the Oswego Lake watershed and affected
8.2 acres. Cases in Tryon Creek affected 3.9 acres, 1.59 acres in Springbrook, and .23 acres in
Willamette River watershed.
There were eleven land uses cases involving sensitive lands or within 50 ft of a waterway. All
eleven land-use cases involved infrastructure crossing a waterway. There were five riparian
delineations in FY2425. Sixteen building permits were issued for properties with sensitive lands
or within 50 ft of a waterway.
Unavoidable crossings (utilities, driveways, etc.) contributed to the disturbance of 26,636 sq ft.
Mitigation or enhancement of 80,163 sq ft was required as a result (an average 3:1 mitigation
ratio). Seven investigations were conducted for sensitive lands violations and resulted in $4,000
in fines. Five RP District violations were issued. Two of these violations were a citation to
encourage compliance..
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B8
Another way that the City protects riparian habitat is through modifications to municipal code.
In FY2425, the Community Development Code were approved in July 2025: clarified a building
cannot cantilever into a 5-foot landscape buffer along an access lane; revised the Native Plant
List to classify Snowberry & Douglas Spiraea as shrubs; and, added a Standard and Enhanced Code
Enforcement Fee for violations in a Sensitive Lands District based upon square footage of
disturbed area. In addition, the city amended code in keeping with the State-mandated Climate-
Friendly & Equitable Communities efforts to minimize impervious area by removing minimum
parking requirements, adding maximum parking requirements, and adopting new "large" surface
parking lot standards if 1/2 acre in size or greater.
B.6.2.2 Enhancement of Riparian Habitat
The City enhances riparian habitat through 1) restoration of City-owned properties, 2) restoration
of private riparian areas through the Habitat Enhancement Program (HEP) and the Audubon
Society’s Backyard Habitat Program, 3) providing technical expertise to the three local watershed
councils operating within the City’s jurisdiction, and 4) implementation of the Parks Master Plan
and the Urban and Community Forestry Plan (UCFP).
B.6.2.2.1 Public Property Restoration
The City contains numerous parks and recreational areas which are maintained by Parks; 460
acres are natural lands and 185 acres are developed parks. Many of these areas are adjacent to
or include streams and directly-connected upland habitat that undergo periodic restoration by
staff. The City is actively restoring 380 acres of public land at 42 sites and spent approximately
$323K on restoration of City-owned land in FY2425. The Parks and Recreation Department is
restoring 16 sites with riparian areas. Staff restored 4.5 acres of wetland in FY2425.
B.6.2.2.2 Private Property Restoration
The City uses the HEP to provide funds to local watershed councils for projects that enhance
privately-owned riparian habitat and provide public education. Applications are prioritized
according to the amount of riparian habitat enhanced, connectivity to City projects or past
private restoration projects, public education hours, and whether the project is in an area
undergoing severe hydromodification as determined by the 2015 Hydromodification Assessment
(City of Lake Oswego, 2015).
The City actively supports the Backyard Habitat Certification Program which is a partnership
between the Columbia Land Trust and the Audubon Society of Portland. The program provides
site visits and advice from professionals on transforming properties into areas with native
habitat. It is a very well-received program by Lake Oswego citizens and, in 2018, the City was 3rd
in the region for program participation. The Backyard Habitat program affects almost 100 acres
in the City. Of that, 24 acres are either in sensitive lands or within 50 ft of a waterway.
Overall, the City has spent $857K on direct restoration of private habitat from FY1718 to FY2324
through the HEP and the Backyard Habitat Program. In FY2425, the City spent $124K through
these programs.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B9
B.6.2.2.3 Master Plans
The City created a Natural Areas Habitat Management Plan that was adopted by the City Council
in December 2022. The plan provides a framework to support the long-term health and resiliency
of the City's natural areas. The plan has eight objectives which range from noxious weed control
to the enhancement of hydrologic functions in riparian areas.
Rassekh Park in the Oswego Lake watershed is being planned as a multi-use area including a skate
park. Current plans for the park include walking areas, an athletic field, and a play area.
Stormwater will be managed by two infiltration raingardens to protect the downstream Pecan
Creek watershed. The Lake Oswego Recreation and Aquatic Center is co-located with the newly
re-designed Golf Course in the Oswego Lake watershed and include stormwater treatment.
There are currently 3 projects the City is working on. One project runs from Oswego Lake Dam to
the Willamette. Removing invasive weeds along creek and planting along banks. Hallinan Creek,
Removing invasive weeds and planting along banks for shade. We have added the property at
1025 Bullock for restoration this year. Glenmorrie Creek has ongoing restoration.
The City updated its Urban Forestry Report in December 2022 and its UCFP in June 2024. The
UCFP provides new metrics to increase, protect, and maintain the urban forest which is measured
with the Urban Forestry Report.
B.6.2.3 Canopy Cover
The City analyzes LiDAR data when new data becomes available in order to provide input to its
Urban and Community Forestry Plan and its TMDL Implementation Plan. The last LiDAR was flown
in the summer of 2024 and the City expects to have full use of the data in 2026.
A LiDAR flight provided data to the City in 2019. The LiDAR data is combined with aerial maps and
layers from the City’s LO Maps to create a snapshot of the city-wide canopy coverage. The
following is a summary of the 2019 LiDAR data analysis.
The 2019 data shows an increased tree canopy city-wide (+5%) from 2014 to 2019 even with the
increased population and subsequent development experienced by the City during the same
period (Parametrix, 2021). The increase appears to be primarily from increased tree growth such
as trees reaching the minimum 10-ft tree height required by the modeling program and trees
experiencing horizontal growth of their canopy (see Figure B.1). For example, the light-green
areas in Figure B.1 without adjacent dark green areas are likely trees that reached a minimum
10-ft height since it is unlikely, but not impossible, that a newly planted tree in 2014 would reach
the 10-ft minimum height.
Areas with the greatest increase in canopy cover (more than 20%) occurred in the Foothills,
Evergreen, and Lakewood neighborhoods (City of Lake Oswego, 2009). Areas with the largest
amount of canopy cover (more than 60%), however, were in the Lakeview and Forest Hills
neighborhoods. Interestingly, the canopy coverage increased by 54 acres from 2014 for right-of-
way areas in the City. This is likely due to the 10-ft minimum scenario discussed in the previous
paragraph.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B10
Within areas designated for recreation and conservation, such as parks, golf courses, or schools,
the greatest increase was seen at schools (10% increase). While parks and other natural areas
saw a small increase (2%), the increase in canopy acreage was substantial (15 acres). Golf courses
(primarily at the Lake Oswego Country Club) saw a decrease in canopy cover.
Canopy coverage was 89% (+2% from 2014) in areas with a Sensitive Lands designation which
indicates that the protections placed on these areas in 2016 may be having an impact. The
greatest increase was in the riparian protection areas (+21 acres), which has the most protection,
as opposed to the resource conservation (+2 acres) or habitat benefit areas (+1 acre).
Figure B.1. Canopy growth between 2014 canopy (dark green) and 2019 (bright green).
Source: Parametrix, 2021
B.6.2.4 Effective Shade
Channel morphology and near-stream vegetation characteristics influence the amount of shade
that can be used to stabilize stream temperatures. For instance, a large tree near a stream is
more effective than a tree 20 ft away but is less effective than 15 smaller trees with interweaving
canopies near a stream. The City completed analyses for the riparian areas throughout the City
and for riparian areas protected by the City’s sensitive lands buffers (Parametrix, 2021).
B.6.2.4.1 Effective Shade Targets
As shown in Figure B.2, the City met the effective shade targets in 2019 only in the Tryon Creek
watershed and only when all riparian areas, protected and unprotected, were included in the
analysis. The largest difference between the effective shade target and the actual effective shade
in all riparian areas continues to be in the Tualatin River (-9%). The smallest difference is in the
Springbrook Creek watershed (-0.7%).
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B11
When the results are confined to the protected riparian areas with a sensitive lands designation,
the effective shade targets were not met in any of the watersheds. Differences between the
target and the actual effective shade in the protected areas ranged from -0.5% in the Tryon Creek
watershed to close to -14% in the Oswego Lake and Tualatin River watersheds.
Figure B.2. Effective Shade (Protected and Non-Protected Areas) by Watershed.
Source: Parametrix, 2021
B.6.2.4.2 Historical Comparison
The effective shade in all riparian areas increased incrementally from 2014 with the exception of
the Tryon Creek watershed (see Figure B.3). The Tryon Creek watershed saw a slight decrease
(1%). The Tualatin River watershed saw the largest increase (1.8%) across the watersheds.
Figure B.3. Effective Shade in Sensitive Lands by Watershed, 2014 and 2019.
Source: Parametrix, 2021
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B12
When the results are confined just to the protected riparian areas (those with a sensitive-lands
designation), both Oswego Lake and the Tualatin River watersheds saw increases greater than
1%; the Tyron Creek watershed still saw a 1% decrease.
In both the 2014 and 2019 effective shade analyses, there was a 5% difference, on average,
between the effective shade in areas protected by the sensitive lands overlay and the effective
shade for all riparian areas (see Table B.4). This difference is due primarily to open channels which
are not covered under the City’s municipal code for sensitive lands but which are actually
historical stream segments that have been channelized and/or otherwise changed significantly
by development. In other words, the City’s effective shade progress would be greater if there
was protection for these areas especially in the Oswego Lake watershed.
Table B.4. Effective Shade Comparison between Protected and Non-Protected Riparian Areas.
Watershed
2014 Effective Shade 2019 Effective Shade
Sensitive
Lands
All Riparian
Areas Difference
Sensitive
Lands
All Riparian
Areas Difference
Tualatin River 76.0% 81.4% 5.4% 77.6% 83.2% 5.7%
Oswego Lake 77.6% 87.6% 10.0% 78.6% 88.5% 9.9%
Willamette River 80.5% 88.2% 7.7% 80.9% 88.8% 7.9%
Tryon Creek 91.7% 93.2% 1.5% 90.7% 92.2% 1.5%
Springbrook Creek 86.2% 89.3% 2.9% 86.5% 89.6% 2.9%
Average 82.4% 87.9% 5.5% 82.9% 88.5% 5.6%
B.6.2.5 Existing Shade Protection
The City uses public education and enforcement of its tree code to stress the importance of tree
canopy on stream temperature. Staff included the importance of tree canopy to stream
temperatures in social media posts in FY2425.
The Planning Department reported that there were no changes in the municipal code that
affected tree canopy. Ten 10 violation letters were issued for removing trees without a tree
removal permit. Fourteen 14 mitigation trees were required for the violations.
B.6.2.6 Cold Water Refugia
The City worked with the USGS, City of Wilsonville, Meyer Memorial Trust, and Benton County
Soil and Water Conservation District to map cold-water refugia (CWR) along the mainstem of the
Willamette River in 2016 and 2017 (Mangano et.al, 2018). Four sites within the City's jurisdiction
were monitored for water temperature: Tryon Creek, Oswego Creek, Glenmorrie Dr seep, and
Arbor Creek seep. Two sites, Tryon Creek and the seep at Glenmorrie Dr, showed some promise
as cold-water refugia with temperatures between 15 and 17° C from June to mid-September.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B13
The seep at Arbor Creek, while somewhat cooler than the Willamette River, neared or exceeded
20°C several times between June and mid-September. Because seeps do not provide a large
volume of water for cold-water refugia, especially in comparison to the Willamette River’s
volume, the City's best option for protecting CWR is Tryon Creek. The City currently works with
the Tryon Creek Watershed Council on riparian restoration and preservation through its Habitat
Enhancement Program. In addition, the City adopted the Natural Areas Habitat Management
Plan which will be implemented by Parks staff and provides for restoration of areas with direct
discharges to the Willamette River.
Another avenue for protecting CWR is to increase summer baseflows. The City’s SWMM requires
developers and City-initiated projects to infiltrate, if technically feasible, the 10-yr 24-hr storm
(3.2 inches).
B.7 ADAPTIVE MANAGEMENT
The City submitted its revised TMDL Implementation Plan in December 2023 and received DEQ
approval on July 24, 2024. The City is not requesting adaptive management for FY2425.
B.8 OTHER ACTIVITIES
This report and the NPDES MS4 report contain almost all of the information regarding the City’s
activities to reduce TMDL parameters for which it has been appointed a DMA. Other activities
that have not been previously discussed in these reports are listed in Table B.5.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B14
Table B.5. Metrics for TMDL Management of Stream Temperatures
Measure Goal Tracking Measure FY2425 Activities
Riparian
Habitat
Quality
Enhance and protect
riparian buffers
Continue habitat restoration efforts on City-
owned riparian buffers. Track the number of
stream miles restored
The Parks and Recreation Department is restoring 16
sites with riparian areas. Staff starting working on 4.5
acres of wetlands in FY2425.
Riparian
Habitat
Quality
Enhance and protect
riparian buffers
Track the acreage that is under active restoration
in City-owned areas. Staff are restoring habitat at 46 sites (400 acres).
Riparian
Habitat
Quality
Enhance and protect
riparian buffers and tree
canopy
Report on the progress of the Natural Areas
Habitat Management Plan and its implementation
including any projects protecting cold water
refugia
There are currently 3 projects the City is working on.
One project runs from Oswego Lake Dam to the
Willamette. Removing invasive weeds along creek and
planting along banks. Hallinan Creek, Removing
invasive weeds and planting along banks for shade. We
have added the property at 1025 Bullock for
restoration this year. Glenmorrie creek, ongoing
restoration.
Riparian
Habitat
Quality
Enhance and protect
riparian buffers
Track changes made to the application process or
prioritization of projects funded through the
Habitat Enhancement Program
The City provides $30K to each watershed council for
riparian restoration projects on private property. The
watershed councils use approximately 3% of the $60K
on stormwater education events or information
presented prior to volunteer work parties.
Riparian
Habitat
Quality
Enhance and protect tree
canopy
Update the canopy cover and effective shade
analyses every 5 years or when new LiDAR
becomes available. Track the amount of increase
or decrease of canopy cover and effective shade
by watershed.
New LiDAR data was collected in the summer of 2024.
Preliminary data will be available in the spring of 2025;
however, the City does not expect to have full use of
the data until 2026.
Riparian
Habitat
Quality
Enhance and protect
riparian buffers and tree
canopy
Track the total acreage of properties participating
in the Backyard Habitat Program.
Track the total acreage of properties participating
in the Backyard Habitat program that have a
sensitive lands overlay or are within 50 ft of a
waterway.
The Backyard Habitat program affects approximately
100 acres in the City. Of that, 34 properties or ~20
acres are either in sensitive lands or within 50 ft of a
waterway.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B15
Riparian
Habitat
Quality
Enhance and protect tree
canopy
Update the Urban Forestry Report every 5 years
or when new LiDAR becomes available. Track
changes in canopy coverage.
The Urban Forestry Report was updated in December
2022 using 2019 LiDAR data. Tree canopy cover
increased from 48.6% in 2014 to 53.4% in 2019 (4.8%
increase). Using the data from the Urban Forestry
Report, staff updated the Urban and Community
Forestry Plan which was adopted in June 2024. The
Plan provides new metrics to increase, protect, and
maintain the urban forest which is measured with the
Urban Forestry Report. The City is beginning a Tree
Code audit.
Riparian
Habitat
Quality
Enhance and protect
riparian buffers
Track the number of private property acres with a
sensitive-lands overlay that have been developed.
Track the number of acres that required
mitigation, e.g. replanting elsewhere
There were 30,034 sq. ft. of RP District disturbed by
unavoidable crossings (e.g., utilities, driveways, etc.)
with 29,046 sq. ft. of RP District mitigation and/or
enhancement.
Riparian
Preservation
Enhance and protect
riparian buffers
Track the number of building permits and land
use cases that affected sensitive lands or areas
that were within 50ft of streams
16 building permits were issued for properties that
intersect with the Sensitive Lands Overlay or are within
50 ft. of a water way. The total acreage is 70 acres
(3,051,558.40 sq. ft.).
Riparian
Preservation
Enhance and protect
riparian buffers Track the number of riparian delineations.
Five delineations were completed in FY2425. Note:
Delineations map an area with natural resources; it
does not result in development.
Riparian
Preservation
Enhance and protect
riparian buffers and tree
canopy
Track the number of code enforcement cases
conducted because of sensitive lands violations.
Track the number of citations issued due to a
sensitive lands violation
Five RP District violations were issued. Two of these
violations were a citation to encourage compliance.
Riparian
Preservation
Enhance and protect
riparian buffers and tree
canopy
Track the number of code enforcement cases
conducted that affected tree canopy and the
number of trees planted as mitigation for the
citation.
Track the number of citations from tree canopy
violations.
Ten violation letters were issued for removing trees
without a tree removal permit. Fourteen mitigation
trees were required for the violations.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2324 Page B16
Post
Construction Monitor impervious area.
Summarize municipal code changes that affected
sensitive lands, increased density, or increased
impervious area.
The following changes to the Community Development
Code were approved in July 2025: clarified a building
cannot cantilever into a 5-foot landscape buffer along
an access lane; revised the Native Plant List to classify
Snowberry & Douglas spiraea as shrubs; and, added a
Standard and Enhanced Code Enforcement Fee for
violations in a Sensitive Lands District based upon
square footage of disturbed area. In addition, the city
amended code in keeping with the State-mandated
Climate-Friendly & Equitable Communities efforts to
minimize impervious area by removing minimum
parking requirements, adding maximum parking
requirements, and adopting new "large" surface
parking lot standards if 1/2 acre in size or greater.
Public
Involvement
Provide opportunities for
public involvement.
Work with neighborhood groups, i.e. Friends
Groups, to restore natural habitat. Track the
number of volunteer events or volunteer hours.
The City works with volunteer groups through its
Friends of Parks program and the Habitat
Enhancement Program. In FY2425, the City sponsored
9 volunteer work parties across the city to plant
approximately 3000 native plants and remove invasive
plants.
Public
Education
Provide stormwater
education to the general
public.
Track the number of messages regarding the
effect of temperature on streams or lakes and the
type of outreach used (e.g. social media).
Staff included the importance of tree canopy to stream
temperatures in social media posts.
Source
Control
Reduce potential pollutants
of concern from entering
waterways.
Track the number of traffic signal, and street
lights that have been converted versus the total
number of each type within City limits. This milestone was completed in FY2223.
City of Lake Oswego NPDES MS4/TMDL Annual Report FY2223 Page B17
B.9 REFERENCES
City of Lake Oswego, 2009, Lake Oswego’s State of the Urban Forest Report.
City of Lake Oswego, 2015, Hydromodification Assessment.
Mangano, JF, Piatt, DR, Jones, KL, and Rounds, SA, 2018, Water temperature in tributaries, off-
channel features, and main channel of the lower Willamette River, northwester Oregon,
summers 2016 and 2017, USGS OFR 2018-1184.
Parametrix, 2021, Effective Stream Shade in Lake Oswego, Internal Technical Memorandum.