Publication Summary

Stream temperature monitoring in 2002 and 2003 in the Wenatchee River Basin showed that current conditions at many locations were warmer than the current and proposed water quality standards. In addition, many locations were found to be cooler than the temperature standards. Under 7Q10 flow conditions with full site-potential mature riparian vegetation, significant temperature reductions of 3.8 degrees C and 1.4 degrees C can be expected for Nason Creek and Icicle Creek respectively, while the Wenatchee River can expect a 0.3 degrees C reduction. Temperatures with improvements under critical (7Q10) conditions are predicted to be less than the threshold for fish lethality of 23 degrees C, but greater than the 18 degrees C Class A and 16 degrees C Class AA numeric criteria in many areas. Temperatures with improvements under average (7Q2) conditions are predicted to have greater benefits to the Wenatchee River and its tributaries.

This analysis uses effective shade as a surrogate measure of heat flux to fulfill the requirements of Section 303(d) for a temperature TMDL.

The Wenatchee River and some of its tributaries -- Chiwaukum Creek, Icicle Creek, Little Wenatchee River, Nason Creek, Mission Creek, and Peshastin Creek -- are included in the 1998 303(d) list for impaired waters for temperature in Washington State.

As part of the Wenatchee River Total Maximum Daily Load (TMDL) study for temperature, the Washington State Department of Ecology conducted field work during 2002-2003. This report presents an analysis of the stream water spatial and temporal temperature patterns of selected streams in the Wenatchee River basin based on instream data and thermal infrared radiation (TIR) surveys from 2002 and 2003. A stream temperature model, QUAL2Kw, was used to investigate possible thermal behaviors of the streams for different meteorological, shade, and flow conditions.

Reductions in water temperature are predicted for hypothetical conditions with mature riparian vegetation and improvements in riparian microclimate. Model simulations performed at 7-day average with 10-year return (7Q10) period flow conditions show that an average reduction of 2.7 degrees C is expected compared with the current conditions. Potential reduced temperatures are predicted to be less than the threshold for fish lethality of 23 degrees C, but greater than 18 degrees C in Class A and greater than 16 degrees 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 the federal Clean Water Act Section 303(d) for a temperature TMDL. Effective shade is defined as the fraction of incoming solar shortwave radiation that is blocked by vegetation and topography 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.

Link to EIM data for User Study ID WENRTMDL