Publication Summary

The 303(d) listings for stream temperature in the lower Skagit River basin include the following creeks: Carpenter, Fisher, Hansen, Lake, Nookachamps, East Fork Nookachamps, Red, Turner, and Otter Pond.

This technical assessment uses effective shade as a surrogate measure of heat flux to fulfill the requirements of the federal Clean Water Act Section 303(d) for a Total Maximum Daily Load for temperature.

In addition to load allocations for effective shade, other management activities are recommended for compliance with water quality standards for water temperature, including measures to promote water-use efficiency and increase groundwater inflows into the streams.

The study area for this Total Maximum Daily Load (TMDL) includes the major tributaries to the lower Skagit River below Skiyou Island. The federal Clean Water Act Section 303(d) listings for impaired stream temperature in the lower Skagit River basin include these creeks: Carpenter, Fisher, Hansen, Lake, Nookachamps, East Fork Nookachamps, Red, Turner, and Otter Pond.

Significant reductions in water temperature are predicted for hypothetical conditions with 100-year-old riparian vegetation, improvements in riparian microclimate, and reductions in channel width. Maximum reductions in water temperature would likely result from a combination of mature riparian vegetation, historic channel complexities, and pre-settlement flow regimes.

Potential reduced temperatures are predicted to be less than the Washington State water quality standard of 18°C for Class A waters in most of the segments evaluated. Those segments not expected to be less than the 18°C are the outlets of Lake McMurray and Big Lake. Surface water temperatures in these two lakes frequently exceed 22°C during the summer.

Natural conditions may exceed the numeric temperature criteria mandated by the water quality standards. In these cases, the antidegradation provisions of those standards apply (Chapter 173-209A-030 WAC). These provisions state that "whenever the natural conditions of said waters are of a lower quality than the criteria assigned, the natural conditions shall constitute the water quality criteria."

This technical study uses effective shade as a surrogate measure of heat flux to fulfill the requirements of Section 303(d) for a temperature TMDL. Effective shade is defined as the fraction of incoming solar shortwave radiation, above the vegetation and topography, that is blocked from reaching the stream surface.

In addition to load allocations for effective shade, other management activities are recommended for compliance with water quality standards for water temperature, including measures to promote efficient water use and increase groundwater inflows into the streams.