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. Maxi- mum 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 tem- perature. 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 8.6 °C higher and 3.4 °C 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 tempera- ture drivers within and among streams and over time. Resume : Dans les cours d'eau, la temperature de l'eau est un important facteur de controle de plusieurs processus internes. Afin de mieux comprendre les processus relies a l'apport d'energie solaire dans les cours d'eau et d'en eva- luer les consequences, nous avons examine la dynamique thermique avant, pendant et apres une experience dans laquelle nous avons ombrage experimentalement une section de 150 m d'un ruisseau d'ordre deux dans la chaine des monts Cascades en Oregon. Les temperatures maximales de l'eau ont decru significativement dans la section ombragee, mais les temperatures minimales et moyennes sont restees inchangees. Les calculs de bilans thermiques avant l'experience ont montre que l'energie solaire a une influence dominante sur la temperature du cours d'eau. L'effet du type de substrat sur la temperature du cours d'eau a pu etre examine separement la ou l'eau coule d'abord sur la roche-mere pour ensuite traverser des substrats alluviaux. Les temperatures maximales dans la section de roche-mere d'amont sont jusqu'a 8,6 °C superieures et 3,4 °C inferieures a celles de la section alluviale d'aval. Une meilleure comprehension des facteurs qui influencent non seulement les temperatures maximales et minimales, mais aussi la variation journaliere de la temperature, permettra d'identifier les sections dans lesquelles la temperature du cours d'eau est plus susceptible d'etre affectee par les changements d'ombrage. Plusieurs contradictions apparentes dans la littera- ture scientifique concernant la temperature des cours d'eau peuvent s'expliquer en considerant les variations dans le temps, ainsi que dans un meme cours d'eau et d'un cours d'eau a un autre, de l'importance relative des differents facteurs qui regissent la temperature des cours d'eau. (Traduit par la Redaction) Johnson 923

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