Variable wildfire impacts on the seasonal water temperatures of western US streams: An empirical study
High severity fires can burn down riparian vegetation that shade streams, stabilize streambank and trap moisture and sediment. Water resources managers are concerned about post-fire effects on streams’ thermal regime and their implications for salmonids. In this study, we assessed wildfire effects on the winter and summer water temperature indices of stream stations at Boulder Creek (Oregon), Elk Creek (Oregon), and Gibbon River (Wyoming) over the three following years. We also employed three statistical approaches (bootstrap, air-water temperature regression analysis, and random forest regression) that utilize local water temperature data to examine different facets of the post-fire change in seasonal stream temperatures while accounting for weather effects. Random forest regression results show that at Gibbon River and Boulder Creek stations, wildfires were related to a significant (p < 0.05) decrease in the average winter mean water temperature (MWT) and accumulated degree days (ADD) by 0.31-0.38 ºC and 29.1-61.9 degree days Celsius (DDC), respectively, relative to that of pre-fire. This was despite the weekly air-water temperature regression analysis indicating a steeper air-water temperature relationship at these stations post-fire. The burning of riparian vegetation at Gibbon River and Boulder Creek stations also corresponded to a 0.4-1.1 ºC increase in the average summer MST and a 32-100 DDC increase in the average summer ADD, relative to that of pre-fire. The fire-related changes in seasonal stream temperature indices across the three-stream stations appeared to be determined primarily by the site’s distance from the burn perimeter. The contrasting wildfire effect on the winter and summer stream temperatures could pose additional challenges for thermally-sensitive fishes that are already imperiled by other stressors in these streams and rivers.