Publication Summary

The Walla Walla River basin contains salmonid fish habitat supporting endangered populations of steelhead and bull trout. Although temperatures in upper Mill Creek above the drinking water diversion stayed cool (under 13.5°C) year round, monitoring shows that temperatures at other tributary locations are warmer than federal Clean Water Act water quality standards.

Cooler summertime water temperatures are predicted for hypothetical conditions with mature riparian vegetation and improvements in riparian microclimate. Temperature reductions ranging from 2.5 - 6.0ºC are expected compared with the current conditions. In addition to improvements in stream shading, other management activities such as increasing channel stability and complexity are recommended.

The study area for the Walla Walla Tributaries Temperature Total Maximum Daily Load (TMDL) includes all major tributaries to the Walla Walla River in Washington State. The Walla Walla River basin contains anadromous fish habitat that supports spring chinook, rainbow/steelhead trout, bull trout, and mountain whitefish.

The federal Clean Water Act Section 303(d) listings for temperature in the study area include the Touchet River, Mill Creek, and 25 additional streams.

The Washington State Department of Ecology (Ecology) conducted field work for this study during 2002-2004. This report presents an analysis of the spatial and temporal stream temperature patterns of selected streams in the Walla Walla River basin based on instream data and thermal infrared radiation (TIR) surveys. A stream temperature model (QUAL2Kw) is 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 using low-flow conditions – representing the 7-day average low flow having a 10-year reoccurrence interval (7Q10) – show that stream temperature reductions ranging from 2.5 – 6.0ºC are expected compared with the current conditions. Potential reduced temperatures are predicted to be less than the threshold for fish lethality of 23ºC, but greater than 18ºC in Class A waters in most of the segments in all streams evaluated.

Potential reduced temperatures in two Class AA streams, the North Fork Touchet River and Wolf Fork Touchet River, are predicted to be less than 18°C but greater than the numeric standard of 16°C for those waters. Although currently supporting salmonids, these high-quality bull trout habitat streams will be very vulnerable to the effects of near-stream development. Future reduction to streamflow, reduction of riparian vegetation, alteration of natural channel, or increase in sediment could increase stream temperatures.

Stream temperatures in the upper reaches of Mill Creek above the drinking water diversion remained under 13.5°C all year round.

In addition to increasing stream shading, other management activities are recommended for compliance with the water quality standards for water temperature. These include measures to increase channel stability and complexity.

Link to EIM data for User Study ID ASTO0003