SPATIAL AND TEMPORAL PATTERNS IN RIVER-AQUIFER EXCHANGES IN SIX RIVERS IN THE COLUMBIA RIVER BASIN

The flow of water between rivers and contiguous aquifers is important for maintaining baseflow in rivers and recharging aquifers, particularly in regions where precipitation and runoff are unevenly distributed through the year. Investigations of basin hydrogeology and gains and losses of streamflow for six rivers in the Columbia River basin (the Snake, Lemhi, Willamette, Deschutes, Spokane, and Methow Rivers) were reviewed to identify spatial and temporal patterns in river-aquifer exchanges in the region. River-aquifer exchanges were found to be spatially concentrated with less than half of a river’s length accounting for most of the gains and losses. Three types of geologic or geomorphic features were associated with the largest exchanges: 1) changes in the thickness and width of aquifers, particularly those formed in unconsolidated sediments; 2) contacts between lithologic units; or 3) channel forms that increase the hydraulic gradient between surface water and ground water (e.g., an incised channel) or intercept a larger fraction of shallow ground-water flow (e.g., a channel traversing from one valley wall to the other). Flow between rivers and aquifers varied over time with increased gains and losses of streamflow commonly observed during high flows. In some reaches, the direction of flow between a river and an aquifer fluctuated during high flows indicating rapid reversals in the hydraulic gradient between the ground and surface waters in response to fluctuations in river stage and ground-water levels. The concentration of river-aquifer exchanges at specific geologic or geomorphic features supports approaches to the investigation, analysis, regulation, or management of water resources based on reach-scale geologic differences within a basin. The fluctuating direction of river-aquifer exchanges during high flows demonstrates the importance of concurrent stage and ground-water level data for inferring the direction of river-aquifer exchanges when they cannot be measured directly.