A statistical model for estimating stream temperatures in the Salmon and Clearwater River basins, central Idaho

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A water-quality standard for temperature is critical for the protection of threatened and endan­ gered salmonids, which need cold, clean water to sustain life. The Idaho Department of Environ­ mental Quality has established temperature stan­ dards to protect salmonids, yet little is known about the normal range of temperatures of most Idaho streams. A single temperature standard for all streams does not take into account the natural temperature variation of streams or the existence of naturally warm waters. To address these issues and to help the Idaho Department of Environmen­ tal Quality revise the existing State temperature standards for aquatic life, temperature data from more than 200 streams and rivers in the Salmon and Clearwater River Basins were collected. From these data, a statistical model was developed for estimating stream temperatures on the basis of subbasin and site characteristics and climatic factors. Stream temperatures were monitored hourly for approximately 58 days during July, August, and September 2000 at relatively undisturbed sites in subbasins in the Salmon and Clearwater River Basins in central Idaho. The monitored subbasins vary widely in size, elevation, drainage area, vege­ tation cover, and other characteristics. The result­ ing data were analyzed for statistical correlations with subbasin and site characteristics to establish the most important factors affecting stream tem­ perature. Maximum daily average stream tempera­ tures were strongly correlated with elevation and total upstream drainage area; weaker correlations were noted with stream depth and width and average subbasin slope. Stream temperatures also were correlated with certain types of vegetation cover, but these variables were not significant in the final model. The model takes into account seasonal tem­ perature fluctuations, site elevation, total drainage area, average subbasin slope, and the deviation of daily average air temperature from a 30-year nor­ mal daily average air temperature. The goodnessof-fit of the model varies with day of the year. Overall, temperatures can be estimated with 95percent confidence to within approximately plus or minus 4 degrees Celsius. The model performed well when tested on independent stream-tempera­ ture data previously collected by the U.S. Geologi­ cal Survey and other agencies. Although the model provides insight into the natural temperature potential of a wide variety of streams and rivers in the Salmon and Clearwater River Basins, it has limitations. It is based on data collected in only one summer, during which tem­ peratures were higher and streamflows were lower than normal. The effects of changes in streamflow on the effectiveness of the model are not known. Because the model is based on data from mini­ mally disturbed or undisturbed streams, it should not be applied to streams known to be significantly affected by human activities such as disturbance of the streambed, diversion and return of water by irrigation ditches, and removal of riparian vegeta­ tion. Finally, because the model is based on data from streams in the Salmon and Clearwater River Basins and reflects climatological and landscape characteristics of those basins, it should not be applied to streams outside this region.