Maximum summer stream temperatures were measured at forty study sites in the agricultural River Raisin catchment to investigate how variation in temperature was related to land use/cover, geology, and hydrology. The range of maximum temperatures, from 23 to 39 °C, was surprisingly large. Extremely high temperatures were due in part to a July heat wave, but the range of temperatures was related largely to differences in riparian shading. Average shading, measured at 50 and 100 m upstream of maximum-mini mum thermometers using a Solar Pathfinder™, explained 58% of the variation in maximum temperatures. Surficial geology type explained the next largest fraction of variation, with warmer temperatures tending to occur at sites located in the flat lakeplain region. Lower temperatures at morainal and outwash deposit sites were interpreted to result from greater groundwater contributions to streamflow.

Land use/cover, evaluated over site catchments, was significantly correlated with maximum temperature, with high temperatures strongly associated with agriculture and low temperatures associated with forest and nonforest cover. Due to intercorrelations o f variables at different scales, the degree to which land use beyond the riparian zone controlled stream temperatures was unclear. Likewise, the lineal extent of riparian forest with 2 km upstream of thermometers was negatively associated with maximum te mperatures, but the distance at which shading had its greatest effect could not be determined. However, the data did indicate that upstream shading was more important for higher velocity streams, presumably due to the greater distance traveled by a parce l of water. The simplest regression model for predicting maximum temperature, with a multiple R2 of 0.70, was based on the Pathfinder measure of local shading and surficial geology type. A second, more complex model based on similar parameters, had a multiple R2 of 0.80. The second model predicts that if riparian forests were allowed to regrow and shading were complete at all sites, the range of maximum temperatures would be reduced to 6 °C, with a high temperature of 29 °C. This study illustrates that, within a b asin dominated by agriculture, narrow strips of riparian vegetation and especially trees can provide enough shading to substantially control stream temperatures. This control may be particularly important for determining the thermal environments of strea ms in areas where groundwater contributions are low.