Water Resources of Idaho
|
Introduction
A water quality standard for temperature is essential to protect threatened and endangered salmonids, which need cold, clean water to sustain life. The Idaho Department of Environmental Quality (IDEQ) has established stream temperature standards to protect salmonids, but temperature data for Idaho streams are scant and the current standards are based on laboratory studies that may not represent the "normal" range of temperatures of most Idaho streams. Furthermore, there are apparent inconsistencies that suggest the problem is complicated -- some streams naturally exceed current temperature standards, even though they are unaffected by human activities. Some of these streams appear to support cold-water aquatic life, even while their temperatures exceed current temperature standards. Clearly, a better understanding of the normal range of Idaho stream temperatures and the relationships between aquatic life and stream temperature is needed.
IDEQ is currently working to revise the Idaho stream temperature standards. The U.S. Geological Survey entered into a cooperative agreement with IDEQ to provide information that would help IDEQ establish stream temperature standards that are realistic, enforceable, and that would truly protect Idaho's aquatic life.
The Salmon and Clearwater River Basins, large areas of which are relatively undisturbed by human activities, provide an ideal setting to determine the most important factors controlling stream temperature. The objectives of this study are to:
- Collect continuous stream temperature data at many sites on large and small, unregulated streams in the Salmon and Clearwater River Basins
- Calculate selected temperature metrics that might apply to new regulatory criteria
- Determine the effects of factors such as vegetation, geography, climate, and other basin characteristics on stream temperature
- Develop a model that can be used to predict stream temperatures in locations where data are not currently available
- Carry out assessments of biological communities at selected sites and relate communities to stream temperature regimes and basin characteristics
Summary of Project Results
Stream temperatures were monitored on an hourly basis for approximately 58 days during July, August, and September 2000, at sites in relatively undisturbed rivers and streams in the Salmon and Clearwater River Basins in central Idaho. The monitored sites and the hydrologic subbasins they represent vary widely in size, elevation, drainage area, and vegetation cover. The data were analyzed for statistical correlations with basin and site characteristics to establish the most important factors affecting stream temperature. Maximum daily average stream temperatures were strongly correlated with elevation and total upstream drainage area. Weaker correlations were noted with average subbasin slope, stream depth, and stream width. Stream temperatures were correlated with certain types of vegetation cover, but these variables were not significant in the final predictive model.
An empirical predictive model based on the stream temperature data was developed. The model predicts daily average temperatures between July 24 and September 10 and takes into account seasonal temperature fluctuations, site elevation, total drainage area, average subbasin slope, and the deviation of daily average air temperature from a 30-year norm. Daily average stream temperatures can be estimated with 95% confidence to within approximately + 4 degrees Celsius. The model performed well when tested on independent stream temperature data previously collected at USGS gaging stations, correctly predicting daily average temperatures within the stated error range.
Project Data
For further information on this study please contact us:
USGS Idaho Water Science Center
230 Collins Road
Boise, Idaho, 83702
phone: 208-387-1300
fax: 208-387-1372
email
