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
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.
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.
Willamette River Basin Temperature TMDL modeling reports from Portland State University and USGS:
Other Documents
Coast Fork Willamette Subbasin
TMDL approved by EPA on May 17, 1996
Columbia Slough Watershed
TMDL approved by EPA on Nov. 25, 1998
Middle Willamette Subbasin - Rickreall Creek
TMDL approved by EPA on April 18, 1994
Molalla-Pudding Subbasin
TMDL approved by EPA on Dec. 31, 2008
Pudding River
TMDL approved by EPA on Dec. 18, 1993
Tualatin Subbasin
Tualatin Subbasin TMDL Revision Approved by EPA on December 14, 2012
TMDL approved by EPA on August 7, 2001
Yamhill Subbasin
TMDL approved by EPA on March 16, 1992
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.
Nov. 12, 2025. Webinar Series 2: Streamside Evaluation Requirements
Elements of a streamside evaluation, example tools and resources available to complete a streamside evaluation, examples of completed streamside evaluations
Dec. 4, 2025, 2-3 p.m.: Webinar Series 3
Reservoir Operator Requirements Willamette Subbasins and Lower Columbia-Sandy Temperature Replacement TMDLs
Presentation: Introduction to reservoir surrogate measure and monitoring requirements: What is a TMDL, reservoir temperature surrogate measure and monitoring requirements, timelines and resources available to support planning
Previous workshops
Sept. 30, 2025: Introduction to the Willamette Subbasins and Lower Columbia-Sandy Temperature Replacement TMDLs
Aug. 3, 2022: TMDL Implementation Planning for Reservoir Operators, Workshop 2 of 2
July 15, 2022: Funding Stormwater and Nonpoint Source Pollution Projects, Clean Water State Revolving Fund
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
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
Dec. 12, 2019
April 3, 2019