USING RADAR FACIES AND HYDRAULIC MEASUREMENTS TO DETERMINE HYDROSTRATIGRAPHIC SETTING OF RECENT BRAIDED DEPOSITS, CENTRAL WASHINGTON STATE

Sedimentary structures record the depositional history of alluvial environments and provide a first-order control on base flow, inflow, and the interaction between phreatic and vadose zones. This study is focused on a portion of the Yakima River, which extends from the central Cascades to its confluence with the Columbia River near Richland, Washington, USA. Anthropogenic changes have modified recharge mechanisms throughout the length of the river and have narrowed the width of the braid plain. A multi-agency effort is currently underway to characterize the hydrogeology, historical locations of channels, and geomorphology of six reaches of the Yakima, with a view to determining controls on critical habitat for salmon. Ground penetrating radar (GPR) has proven three-dimensional subsurface mapping capabilities that have wide ranging applications in hydrogeologic investigations. This study focuses on establishing a GPR-based, three-dimensional (3D) model of a side stream alluvial channel and integrating this model with hydrogeologic data including water level measurement and sediment cores.

We have gathered profile-based and three-dimensional (3D) GPR data in one reach of the Yakima river that is particularly well studied, with 8 instrumented boreholes in an ~1 ha site. GPR data from the site were gathered using step-method data acquisition. Riverbank exposures and cores indicate that site geology is dominated by sand to silt size framework surrounding cobble- to pebble-size clasts. Penetration of up to ~12 m was achieved.

Using the radar facies approach, we developed a 3D stratigraphic model integrating the 3D GPR data and ground truth. In the model, we are able to tentatively distinguish between channel, overbank, bar deposits. We compare the preliminary stratigraphic model to successive flow net maps in order to estimate the extent of stratigraphic control on subsurface hydraulic flow. The comparison shows that the GPR derived images explain variation in groundwater flow with greater accuracy than can be obtained using conventional borehole data alone.