TMDL Title: Snake River- Hells Canyon TMDL and WQMP
Water Quality Limited Parameters: total mercury, methylmercury
Pollutants: total mercury, methylmercury
Status: In development
Project area: The project area includes the entire extent of the Snake River and Hells Canyon reservoir complex from the Oregon-Idaho border near Adrian and rivermile 409, to the stateline between Oregon, Idaho, and Washington at rivermile 176.
Project summary
DEQ is developing a mercury Total Maximum Daily Load, or TMDL, for the Snake River-Hells Canyon to minimize the human health risk of mercury exposure from eating fish. The Snake River does not meet water quality standards for methylmercury, an extremely toxic form of mercury that may accumulate in fish.
The scope of this TMDL will focus on a portion of the Snake River on the Oregon-Idaho border. There are three privately owned hydroelectric dams in the project area; Brownlee Dam, Oxbow Dam, and Hells Cayon Dam, which collectively are known as the Hells Canyon Complex.
DEQ establishes water quality criteria to protect beneficial uses of waterways, such as fishing and support of fish and aquatic life. Oregon is required to establish a TMDL for waters violating criteria that protect beneficial uses. A TMDL for each pollutant is determined through an analysis to calculate the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards.
For more information and to review important studies on mercury and methylmercury in the Hells Canyon Complex please visit the USGS Mercury Cycling Hells Canyon Complex website.
Other TMDLs under development in the Snake River include a temperature TMDL replacement project.
DEQ is working on a temperature TMDL update for the Snake River and Hells Canyon Complex reservoirs. Once approved, the updated TMDL will replace the temperature-related allocations, implementation requirements, and supporting analysis in the June 2004 Snake River – Hells Canyon TMDL and Water Quality Management Plan.
Other requirements, including requirements related to bacteria, nutrients, nuisance algae, dissolved oxygen, pesticides, pH, sediment and total dissolved gas, remain in effect under the 2004 TMDL.
Visit the Temperature TMDL Replacement page for more information.
The 2004 TMDL and Water Quality Management Plan for nutrients, dissolved oxygen, pesticides, sediment, temperature and total dissolved gas was approved by EPA on Sept 9, 2004. More information can be found below:
The Snake River Modeling Quality Assurance Project Plan describes the data analyses and modeling planned for development of the Snake River Mercury TMDL. Information presented here adheres to DEQ's and the EPA's guidance for developing, testing, and applying models for TMDL development. DEQ distributed a draft plan in November of 2025 to the Nez Perce Tribe and the Technical Advisory Committee for comments as part of the TMDL development process. The plan and comments are listed below.
For more information and to review important studies on mercury and methylmercury in the Hells Canyon Complex please visit the USGS Mercury Cycling Hells Canyon Complex website.
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