Local and Regional Scale Simulation of River Aquifer Interactions.

Overdraft of groundwater in Sacramento County, California has severely depressed regional groundwater levels and depleted base flows along the Cosumnes River. These developments continue to negatively impact riparian vegetation and threaten the fall migration of endangered Chinook salmon. In this study river aquifer interactions are investigated along a 200m reach of the Cosumnes River using continuous field measurements of riverbed temperature, water content and matric potential to a depth of 10m below the riverbed. These data demonstrate complex seepage patterns including fracture flow and the formation of perched aquifers due to geologic heterogeneity. Riverbed sediment heterogeneity was characterized using geostatistical indicator simulations based on continuous core data. A 3-dimensional reach-scale model was developed using the TOUGH2 simulator. Modeling results suggest that the success of riparian vegetation may depend on the presence of perched aquifers. The reach-scale model will also provide better estimates of riverbed hydraulic properties for use in a larger regional-scale groundwater flow model. The regional model was developed based on detailed geostatistical simulations of the Cosumnes alluvial fan hydrostratigraphy using a version of MODFLOW that includes the ability to simulate unsaturated flow between the river and the aquifer. The regional model is used to assess management scenarios that are developed to improve fall flows and to enhance overall river ecology.