Controls of summer stream temperature in the Pacific Northwest

Abstract

Summary This is a regional study of the landscape, stream, and channel factors that control summer stream temperatures in the Pacific Northwest. The study focusses on two characteristics of stream temperature: the thermal regime, defined as the pattern of seasonal and spatial stream temperature variation in a stream, and the thermal sensitivity, considered here to be the slope of the stream–air temperature relationship. The baseflow index, or degree of groundwater influence in a stream, is hypothesized to be an important regional control of both thermal regime and thermal sensitivity. Weekly stream temperatures were calculated and evaluated at 104 primarily unregulated sites that were geographically distributed across the region. Air temperature and streamflow explained an average of 94% of the variance in annual thermal regimes and 68% of the variance in August thermal regimes at individual sites. Regression results showed that thermal sensitivity to increasing air temperatures was less in summer than at other times of the year. Future climate projections of summer stream temperature, based on thermal sensitivities, are small in comparison to the range of summer stream temperatures that exist across the region. This observation highlights the need to understand summer thermal regimes as well as summer thermal sensitivities when assessing climate change impacts to regional stream temperatures. Multiple regression analysis identified several landscape factors and stream characteristics that explained summer stream temperatures and thermal sensitivities regionally. Two of the most important factors were baseflow index and stream channel slope. Lastly, the summer stream/air temperature ratio of a stream may be useful in assessing existing or expected stream temperature conditions at a site in relation to these factors or in identifying unique environmental conditions or potential restoration needs at a site.