Willamette Basin

The Willamette Basin encompasses 12 subbasins: Lower Willamette, Tualatin, Molalla-Pudding, Yamhill, Clackamas, South Santiam, North Santiam, Middle Willamette, McKenzie, Coast Fork Willamette, Middle Fork Willamette and Upper Willamette. This geographic area comprises the broad Willamette River valley, which is flanked by the forested slopes of the Coast and Cascade mountain ranges. The Willamette River and its tributaries support a wide variety of ecosystems and habitats including forested and depressional wetlands, riparian forests and shrublands, upland and wet prairies, chapparal, woodlands and oak savanna.

The rivers, streams and lakes in the Willamette Basin support native fish and other wildlife. Threatened native populations of Chinook salmon, steelhead trout and bull trout, as well as rainbow and cutthroat trout, and other aquatic life are culturally and economically significant to the basin.

Forestry, agriculture and urban uses dominate land use in the Willamette Basin. While forestry use is active from the higher elevations to the foothills of the Coast and Cascade mountain ranges, agriculture is the largest land use in low elevation valleys.

TMDLs and Water Quality Management Plans

Oregon is required to establish Total Maximum Daily Loads for streams segments which do not meet water quality standards. The TMDL identifies the level of pollutants that a water body can absorb and still meet water quality standards. TMDLs take into account pollution from all sources, including discharges from industry and sewage treatment facilities; runoff from farms, forests and urban areas; and natural sources. TMDLs also include a safety margin to account for uncertainty.

This information is then used to determine what changes must take place to achieve water quality standards. The TMDL will be used to determine whether changes are needed for wastewater discharge permits for industries and sewage treatment facilities. Water quality management plans are also developed based on the TMDLs. These plans document the ways that local landowners, agencies, forest and agricultural land managers (including federal agencies), DEQ and others will implement a specific TMDL and work to improve water quality.

More information on the Willamette

Willamette Toxics Reduction Partnership and Story Maps
Map: TMDL Development Status for 303(d) Listed Waters
Water Quality Management Plans:

TMDL Implementation Plans: Guidelines, Tools and Reporting

Willamette Basin Mercury TMDL Revision

Oregon DEQ and the U.S. Environmental Protection Agency are working together to revise the 2006 Willamette Basin Mercury Total Maximum Daily Load to meet the fish tissue methylmercury criterion adopted in 2011. This criterion allows Oregonians to safely consume higher amounts of fish (approximately 23 8-oz fish meals a month) caught in Oregon's waterways.

On July 3, 2019, DEQ released a draft revised mercury Total Maximum Daily Load and Water Quality Management Plan for the Willamette Basin. Public comments are due by 5 p.m. on Sept. 3, 2019. See the Advisory Committee web page for more information.

The Willamette River Basin is home to seventy percent of Oregon's population. Those who live or work in the basin depend on the river for many resources, and also contribute to potential pollution problems that come with any residential, municipal, industrial, or agricultural operation.

For the amount of land area in the basin, more water flows from the Willamette River than from any other major river basin in the United States. The basin, 180 miles long and 80 miles wide, is bordered by the crest of the Cascade Mountains to the East, the crest of the Coast Range to the West, and the Columbia River to the North where the two rivers meet. About 2.3 million people live in the river basin near its 16,000 miles of rivers and streams, and more than half of them live in the Portland metropolitan area.

As population increases, and land conversion to urban and industrial uses continues in the basin, these changes affect the Willamette River.

For example, the household chemical products applied in and around the home, including pesticides and fertilizers, may end up in the Willamette after passing through treatment plants or in storm water runoff. Pollutants can reach the river through groundwater as well as from runoff and pipes.

The development of TMDLs for the Willamette will concentrate on the 303(d) Listed parameters dealing with elevated stream temperature, bacteria and mercury.

Decades of work and millions of dollars of investment by the State, industry, and cities has reversed some of the worst damage to the Willamette River. Cities and industries began treating wastewater in the 1950s, and treatment has improved steadily since. Flood control reservoirs built by the federal government have increased summer flow, providing waste dilution during this critical period. Today the river is cleaner and healthier than it once was for people and fish. However, there is still much work to be done.

The Oregon Plan is a commitment from Oregon residents, businesses, agencies and governments to work together to ensure our children will inherit healthy watersheds. The Oregon Plan is people working locally; watershed councils coordinating the work; local landowners and governments initiating new ways of doing things; funding and expertise from state, tribal and federal agencies, and businesses and industries; and implementing existing laws and regulations. Most of all, it is a spirit of volunteerism and stewardship characteristic of Oregon and Oregonians.

Efforts to restore the watershed involve everyone in the watershed. Actions include planting vegetation to reduce erosion and keep water cool; changing habits at home, at work, and at play to prevent or reduce pollutants entering waterways; improving fish passage and opening habitat that was blocked by past practices; and reducing erosion and sediment entering streams.

Mercury is another key issue that people can learn more about to reduce its impairment on the environment. DEQ is actively involved in a study to reduce mercury pollution in the Willamette.

Fact Sheets

Overview Maps

Subbasins:

Mercury TMDL reissued by the U.S. Environmental Protection Agency on Feb. 4, 2021

EPA established this TMDL and provided it to the State for implementation. DEQ’s 2019 WQMP for Mercury is in effect.

Willamette Basin Mercury 2019

Addendum 1: Modifications to the Willamette Basin Temperature TMDL

Approved by the US Environmental Protection Agency on November 23, 2011

TMDL approved by the US Environmental Protection Agency on Sept. 29, 2006

Willamette Basin:

The Willamette Subbasins TMDL for temperature was adopted by the Environmental Quality Commission into rule on Aug. 6, 2024. EPA approved the TMDL on Sept. 12, 2024.

EQC adopted an amended Willamette Subbasins TMDL and WQMP rule on May 8, 2025, to include the Willamette mainstem and major tributaries project area.

The amended 2025 Willamette Subbasins temperature TMDLs replaces existing temperature TMDLs included in the 2006 Willamette Basin TMDL and the 2008 Molalla-Pudding Subbasin TMDL.

TMDL Order Memorandum - Sept. 21, 2006
Executive Summary
Acknowledgements
Chapter 1: Overview
Chapter 2: Willamette Basin Bacteria TMDL
Chapter 3: Willamette Basin Mercury TMDL
Chapter 4: Temperature-Mainstem TMDL and Subbasin Summary
Chapter 5: Lower Willamette Subbasin TMDL
Chapter 6: Clackamas Subbasin TMDL
Chapter 7: Middle Willamette Subbasin TMDL
Chapter 8: North Santiam Subbasin TMDL
Chapter 9: South Santiam Subbasin TMDL
Chapter 10: Upper Willamette Subbasin TMDL
Chapter 11: McKenzie Subbasin TMDL
Chapter 12: Middle Fork Willamette Subbasin TMDL
Chapter 13: Coast Fork Willamette Subbasin TMDL
Chapter 14: Water Quality Management Plan
Appendix A: Bacteria
Appendix B: Mercury
Appendix C: Temperature
Response to Public Comments

Willamette River Basin Temperature TMDL modeling reports from Portland State University and USGS:

Coast Fork Willamette Subbasin

TMDL approved by EPA on May 17, 1996

Total Maximum Daily Load

Columbia Slough Watershed

TMDL approved by EPA on Nov. 25, 1998

Middle Willamette Subbasin - Rickreall Creek

TMDL approved by EPA on April 18, 1994

Total Maximum Daily Load

Molalla-Pudding Subbasin

TMDL approved by EPA on Dec. 31, 2008

TMDL Order Memo - Dec. 8, 2008
Executive Summary
Chapter 1: Overview
Chapter 2: Temperature
Chapter 3: Bacteria
Chapter 4: Pesticides
Chapter 5: Nitrate
Chapter 6: Metals
Chapter 7: Water Quality Management Plan
Appendix A: Temperature Analysis
Appendix B: Willamette Near Stream Land Cover
Appendix C: Heat Source Model
Appendix D: Example Effluent Temperatures
Appendix E: Natural Thermal Potential Estimation
Appendix F: Bacteria Analytical Methods
Appendix G: Bacteria Data Review
Appendix H: Bacteria Statistical Test
Appendix I: Current Pesticides
Appendix J: Pesticide Analysis
Appendix K: TSS Turbidity Model
Appendix L: Nitrate Analysis
Appendix M: Metals Analysis
Response to Public Comment

Pudding River

TMDL approved by EPA on Dec. 18, 1993

Total Maximum Daily Load

Tualatin Subbasin

Tualatin Subbasin TMDL Revision Approved by EPA on December 14, 2012

TMDL Order Memo - Aug. 28, 2012
EPA Approval Letter
Fact Sheet: Working for Clean Water in the Tualatin River Subbasin
Executive Summary
Chapter 1: Overview and Background
Chapter 2: pH and Chlorophyll a (Total Phosphorus) TMDL Amendment
Chapter 3: Amendment to 2001 Waste Load Allocations for Ammonia Under 2001 Dissolved Oxygen TMDL for the Tualatin Basin
Chapter 4: Water Quality Management Plan
Appendix 2-A: Tualatin River Total Maximum Daily Loads: Total Phosphorus and Dissolved Oxygen Analysis for the Upper River Final Report
Appendix 2-B: Tualatin River Total Maximum Daily Loads for Total Phosphorus (4/20/2009) Phosphorus Control Period
Appendix 3-A: Changes to the 2001 Dissolved Oxygen TMDL for the Tualatin Basin
Response to Comments

TMDL approved by EPA on August 7, 2001

Total Maximum Daily Load and Water Quality Management Plan
Appendix A: Temperature Technical Analysis
Appendix B: Bacteria Model Description
Appendix C: Supporting Documents for the Chlorophyll A/pH (Phosphorus) TMDL
Appendix D: Supporting Documents for the Dissolved Oxygen TMDL
Appendix E: Low pH Discussion
Appendix F: Tualatin Subbasin Fish Habitat and Fish Community Information
Appendix G: Toxics Discussion
Appendix H: Biological Criteria Discussion
Appendix I: Tualatin Subbasin Water Quality Management Plan
Response to Public Comment

Yamhill Subbasin

TMDL approved by EPA on March 16, 1992

Total Maximum Daily Load

TMDL Report In-Progress (Data analysis and report writing phase) - For updated TMDL.

DEQ’s Willamette Basin Coordinators routinely host workshops on timely topics in TMDL Implementation. Below are links to recent workshops and upcoming dates.

If you would like to receive updates by email about these workshops, please email the basin contact above for your region.

Aug. 3, 2022: TMDL Implementation Planning for Reservoir Operators, Workshop 2 of 2, 9 a.m. - 11:30 a.m.

Agenda flyer

Join the meeting via Zoom

July 15, 2022: Funding Stormwater and Nonpoint Source Pollution Projects, Clean Water State Revolving Fund

Meeting recording

July 7, 2021: TMDL Implementation Planning for Reservoir Operators, Workshop 1 of 2

June 1, 2021

TMDL Planning and Implementation for Counties

June 8, 2021
TMDL Planning and Implementation for Responsible Persons, Water Conveyance Entities

May 25, 2021

TMDL Planning and Implementation for Permitted MS4s

Agenda
Presentation: TMDL Planning and NPS Implementation for DMAs with MS4 Permits
Workshop Recording

May 11, 2021
TMDL Planning and Implementation for Non-permitted Stormwater Management

May 4, 2021

Protecting Water Quality in the Willamette Basin: An Introduction to the Total Maximum Daily Load Program for New Designated Management Agencies and Responsible Person