TMDL Title: Snake River temperature TMDL
Water Quality Limited Parameters: Temperature
Schedule: DEQ will convene a TMDL local advisory group with the first meeting occurring in 2026. The TMDL will be submitted to EPA in 2027. EPA must take action to approve or disapprove the TMDL by Dec. 4, 2028, as required by court order.
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, Oregon and river mile 409, to the state line between Oregon, Idaho, and Washington at river mile 176.
Project summary
The Snake River originates in the Greater Yellowstone Area in Wyoming and runs north across the Oregon-Idaho border until meeting with the Columbia River. The scope of this TMDL will focus on a portion of the Snake River from where it meets the Oregon-Idaho border to the Washington-Oregon-Idaho border. There are three privately owned hydroelectric dams in the project area; Brownlee Dam, Oxbow Dam, and Hells Canyon Dam, which collectively are known as the Hells Canyon Complex.
Monitoring data show temperatures in the Snake River are too warm for sensitive fish and do not meet Oregon's EPA approved temperature standards. As a result, the Snake River and Hells Canyon Complex reservoirs have been placed on a national list of impaired waters. The federal Clean Water Act requires states or EPA to develop a
Total Maximum Daily Load, or TMDL, for each water listed as impaired. Once approved, the updated TMDL will replace the temperature-related requirements in the June
2004 Snake River – Hells Canyon TMDL and Water Quality Management Plan.
DEQ is replacing the 2004 Snake River Hells Canyon temperature TMDL following the outcome from two court cases and related EPA actions. In response to a 2012 court decision, EPA disapproved Oregon's
Natural Conditions Criterion for temperature in 2013, which had been used in most temperature TMDLs from 2003 through 2012. A 2019 court ruling then required DEQ and EPA to replace 15 TMDLs using the remaining approved parts of Oregon's temperature water quality standard.
Quality Assurance Project Plan: Snake River
A Quality Assurance Project Plan is required prior to writing the TMDL. The plan outlines the framework for developing a temperature TMDL, including defining the problem scope, key temperature-related processes and technical modeling approach. It also details the data sources, evaluation methods, potential management scenarios and project management components such as documentation and team coordination.
Public participation
TMDL development and implementation is a public process which includes the establishing of a local advisory group, holding informational webinars and a public comment period.
Local advisory group
DEQ seeks public involvement by establishing a local advisory group, which provides technical input to DEQ. The local advisory group meetings are public and community members and organizations may attend. DEQ will convene a TMDL local advisory group with the first meeting occurring in 2026. Public webinar, hearing and comments
DEQ will also hold an informational webinar and a public hearing and comment period, where anyone can provide comments on the draft TMDL. DEQ makes responses available to the public following the comment period.