Publication Summary

Cooler water temperatures are predicted for hypothetical conditions with mature riparian vegetation, improved microclimate, reduced width, and increased groundwater inflows. Potential maximum temperatures could be less than the threshold for lethality of 23°C but greater than the water quality standards in most of the stream segments that were evaluated.

This technical assessment uses shade as a surrogate measure of heat flux. In addition to load allocations for shade, other management activities are recommended for compliance with water quality standards for water temperature, including measures to reduce channel widths.

The 1,770 km2 Stillaguamish River basin contains about 1,400 kilometers of anadromous salmon habitat. The 303(d) listings for stream temperature in the basin include Deer Creek, Higgins Creek, Little Deer Creek, Pilchuck Creek, the mainstem Stillaguamish River, North Fork Stillaguamish River, and South Fork Stillaguamish River.

Substantial reductions in water temperature are predicted for hypothetical conditions with mature riparian vegetation, improvements in riparian microclimate, reduction of channel width, and increases in groundwater inflows. Potential reduced temperatures are predicted to be less than the threshold for lethality of 23°C but greater than 18°C in Class A and greater than 16°C in Class AA waters in some or most of the segments in all streams that were evaluated.

This technical assessment uses effective shade as a surrogate measure of heat flux to fulfill the requirements of Section 303(d) for a Total Maximum Daily Load (TMDL) for temperature. Effective shade is defined as the fraction of incoming solar shortwave radiation that is blocked from reaching the surface of the stream.

In addition to load allocations for effective shade, other management activities are recommended for compliance with the water quality standards for water temperature including measures to reduce channel widths.

Link to EIM data for User Study ID GPEL0009