Patterns and sources of thermal heterogeneity in small mountain streams within a forested setting

Spatial thermal patterns and the sources of those patterns were examined in four mountain streams in northeast Oregon and Idaho during hot summer days in 2001. Summer baseflow ranged from <0.007 to 0.22 m3/s. Maximum and minimum temperatures increased in the downstream direction, with the diurnal range decreasing with increasing stream size. Each stream was thermally diverse spatially, diurnally, and seasonally, with unique landform and geologic influences on the daily temperature maxima. Insolation was the most important predictor for maximum temperature at each stream. Minimum daily temperatures were less variable than maxima and followed a seasonal and elevation gradient, with minimum air temperature a more important predictor than either the insolation from the previous day or channel features. Models for each stream using insolation and physical habitat characteristics explain 32 to 67% of maximum temperature and 17 to 51% of minimum temperature variability. Groundwater inputs moderated thermal conditions, particularly local maxima, but did not subsume the predominant temperature range controls of insolation and air temperature.