THREE-DIMENSIONAL HYDROGEOLOGY OF THE RUSSIAN RIVER WATERSHED, CALIFORNIA

A three-dimensional (3D) hydrogeologic model of the Russian River watershed is being developed to help evaluate the water resources in Sonoma and Mendocino Counties, California. This model translates the conceptual hydrogeologic framework, based on geologic and geophysical data, into a digital 3D computer framework. The 3D framework serves as the geologic input into a hydrologic flow model for watershed-scale analysis of water quantity and quality. Three sedimentary basins in the watershed are modeled (Potter Valley, Ukiah-Redwood Valley, and Alexander Valley), each of which is separated, and underlain, by a heterogeneous assemblage of basement rocks. Valley fill within each basin ranges from 100 to more than 400 meters (m) thick and is composed of: 1) Holocene channel alluvium, 2) dominantly Pleistocene unconsolidated sediment, 3) Pliocene consolidated sediment, and, in the southern third of the watershed, 4) Pliocene and Miocene Sonoma Volcanics. To construct the framework model, surface and subsurface hydrogeologic data were compiled from geologic maps, drillers’ lithologic descriptions, borehole geophysical logs, and a depth-to-basement model based on gravity. The data were then interpolated using geologic modelling software to generate 3D surfaces and extents of each hydrogeologic unit. In the model, the channel alluvium ranges in thickness from 20-50 m along the Russian River within the sedimentary valleys. Young unconsolidated sediment (<100 m thick) is present throughout most of the three basins, overlying either Pliocene consolidated materials or the Sonoma Volcanics. Pliocene consolidated sediments (50-250 m thick) are found in each basin and are the basal unit in Ukiah-Redwood and Potter Valleys. The Sonoma Volcanics (~100 m thick) are limited to Alexander Valley and are likely to interfinger with consolidated sediments based on relations observed in nearby outcrops. The 3D model provides insight into the geologic processes that shaped the Russian River watershed and how groundwater may flow in the watershed.