TMDL Title: Snake River mercury TMDL
Water Quality Limited Parameters: Total mercury, methylmercury
Pollutants: Total mercury, methylmercury
Status: Rulemaking under 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 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.
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.
Rulemaking
TMDL development and implementation is a public process. DEQ will develop this TMDL by rule per Oregon Administrative Rule Chapter 340 Division 42. During a rulemaking process, DEQ seeks public involvement by holding public hearings, establishing rule advisory committee meetings for technical and policy input, and for feedback regarding potential fiscal impacts related to the rule.
Snake River mercury TMDL rulemaking
DEQ will hold rule advisory committee meetings and a public hearing. DEQ will present the draft TMDL and Water Quality Management Plan to the Environmental Quality Commission, DEQ's formal policy and rulemaking body, for adoption before submitting the TMDL to EPA for approval or disapproval.
DEQ has a rulemaking website for rule advisory committee meeting dates and information, draft project documents including the TMDL, Water Quality Management Plan, Technical Support Document, and Fiscal Impact Statement, and to sign up for email updates. Please check the rulemaking webpage for updates.
Want to get updates about the Snake River mercury TMDL? Sign up for
email updates on this topic.