On August 13, the Environmental Protection Agency (EPA) reissued the Columbia and Lower Snake Rivers Temperature Total Maximum Daily Load (TMDL), which reflects revisions to EPA’s May 18, 2020 Draft TMDL, following a public comment period.

A TMDL represents a total cap on pollutants that may be discharged to a water body or certain segments thereof, which is then divided among wasteload allocations for point sources (which are covered by National Pollutant Discharge Elimination System (NPDES) permits), load allocations for nonpoint sources — including dams — and a margin of safety. Pursuant to Section 303 of the Clean Water Act, a TMDL was required for the Columbia and Lower Snake rivers because significant portions of each have been identified by the states of Washington and Oregon as impaired due to temperatures that exceed the states’ water quality standards. The Columbia and Lower Snake rivers are home to certain fish species, including salmon and steelhead, that are particularly sensitive to temperature changes. EPA’s TMDL explains that dams on the Columbia and Lower Snake rivers tend to have a cumulative warming impact on the mainstem rivers during summer months, despite cold water releases in the upper portion of the Lower Snake River.

EPA’s reissued TMDL includes several key findings, including:

  • Temperature impairments are primarily due to the cumulative impacts of dam impoundments and climate change, and are widespread throughout the Columbia and Lower Snake rivers;
  • Temperature reductions of up to 3.1°C in the Lower Columbia River and up to 2.8°C in the Lower Snake River are necessary to meet water quality standards;
  • The “allowable thermal loading capacity” of the Columbia and Lower Snake rivers is 0.3°C, which is divided equally among dams, point sources, and tributaries;
  • A reserve allocation for NPDES point sources is included to account for future growth, new sources of pollution, and adjustments to wasteload allocations for existing facilities;
  • The TMDL identifies 23 tributaries in the Lower Columbia River that provide cold water refuge for migrating salmon and steelhead, and the TMDL sets temperature, flow, and cold water volume targets within these tributaries to increase available cold water refuge.

EPA’s August 2021 TMDL is largely unchanged from the May 2020 version. However, there are some significant differences in the revised TMDL, including tribal water quality standards for the Spokane Tribe of Indians and the Confederated Tribes of the Colville Reservation. EPA also revised the TMDL to remove references to use attainability analyses, which the May 2020 TMDL suggested the states employ to consider changes to designated uses in areas where water quality standards were not met. However, in response to comments expressing concern that EPA was encouraging the states to weaken their water quality standards, EPA removed the reference to use attainability analyses. The reissued TMDL also clarifies that target temperatures will apply at all locations within the mainstem Columbia and Lower Snake rivers, including target sites in dam tailraces, as well as fish ladders, dam forebays, surface waters in impoundments, and nearshore zones.