A HEAT BUDGET FOR A SHALLOW CHANNEL/FLOODPLAIN AQUIFER: INFLUENCE OF RIPARIAN ZONE LAND USE ON RIVER TEMPERATURE, NORTHEASTERN OREGON

Efforts to restore salmonid fisheries in the Umatilla River in Northeast Oregon are challenged by summer stream temperatures that are higher than historically recorded and detrimental to the fish. Earlier analyses suggest that river temperatures are cooled and buffered by river/floodplain groundwater exchange. This work attempts to develop a heat budget for the floodplain/riparian zone groundwater system including the influence of surface conduction, a factor altered by floodplain shading. Two reaches of the river and associated floodplain aquifer were instrumented with over 70 ground water monitoring wells and hundreds of temperature loggers. This work showed that the temperature of the infiltrating river water and heat conduction from the land surface drive ground water temperature. The importance of either component was assessed with numerical simulations (VS2DHI) of heat and water flux and is dependant upon the thicknesses of the vadose zone and aquifer, and the residence time of the ground water. In cases where surface conduction dominates, land cover can affect the average ground water temperature, which in turn influences stream temperatures.