Factors influencing stream temperatures in small streams: substrate effects and a shading experiment

The temperature of stream water is an important control of many in-stream processes. To better understand the processes and consequences of solar energy inputs to streams, stream temperature dynamics were examined before, during, and after experimental shading of a 150-m reach of a second-order stream in the Oregon Cascade Range. Maximum water temperatures declined significantly in the shaded reach, but minimum and mean temperatures were not modified. Heat budget calculations before shading show the dominance of solar energy as an influence of stream temperature. The influence of substrate type on stream temperature was examined separately where the water flowed first over bedrock and then through alluvial substrates. Maximum temperatures in the upstream bedrock reach were up to 8.6degreesC higher and 3.4 degreesC lower than downstream in the alluvial reach. Better understanding of factors that influence not only maximum but minimum temperatures as well as diurnal temperature variation will highlight types of reaches in which stream temperature would be most responsive to changes in shading. Many apparent discrepancies in stream temperature literature can be explained by considering variation in the relative importance of different stream temperature drivers within and among streams and over time.