PRELIMINARY WATER AND HEAT BUDGETS FOR TWO FLOODPLAIN AQUIFERS ALONG THE UMATILLA RIVER IN NORTHEAST OREGON

Characterizing the thermal regime of relatively pristine streams is an important first step in developing temperature standards that adequately protect salmonid habitat. An ongoing project, Data-Rich Decision Support Environment for the Development of Water Temperature Standards for Salmonid Habitat in the Pacific Northwest (NASA), is combining remote sensing, ground-based data collection and numerical modeling to decipher the factors controlling the thermal complexity within the Umatilla River floodplain in northeastern Oregon.

Two floodplain nodes along the Umatilla River corridor have been intensively instrumented with ~ 90 groundwater monitoring wells and > 350 sensors measuring surface water and groundwater temperature and chemistry. Thermal, geochemical and hydrogeologic data are being used to identify sources of water to floodplain aquifers and quantify groundwater – surface water interaction. Hydraulic gradients are primarily perpendicular to the floodplain axis and high-resolution temporal data indicate that the alluvial aquifer reacts quickly to changes in river stage and to a lesser degree, temperature. Preliminary analyses suggest that stream water is the dominant source of water to and driver of temperature in the floodplain aquifers, with smaller and locally important contributions from regional bedrock and hillslope aquifers. Mechanisms controlling thermal flux are being investigated by conducting heat push-pull tracer tests, analyses of natural surface water exchange with floodplain aquifers and numerical modeling.