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i Immediately below the Modesto and Riverbank Formations is the Pleistocene-age Turlock Lake Formation, consisting of low-permeability, fine-grained siltstone and claystone with interbedded and intermittent coarse-grained sand zones present as channel sands and gravels that have extremely high permeabilities. This unit is derived almost principally from the Sierran granitic 5 rock and formed under a flood-plain environment. �1 3.3 Site Hydrogeology 1 j+ The San Joaquin Valley comprises the southernmost portion of the Great Valley Geomorphic Province of California(Figure 1). The Great Valley is a broad structural trough bounded by the tilted block of the Sierra Nevada on the east and the complexly folded and faulted Coast Ranges on the west. The Eastern San Joaquin Subbasin is defined by the areal extent of unconsolidated II to semiconsolidated sedimentary deposits that are bounded by the Mokelumne River on the north and northwest; San Joaquin River on the west; Stanislaus River on the south; and consolidated bedrock on the east(Figure 2) . The Eastern San Joaquin Subbasin is bounded on the south, southwest, and west by the Modesto,Delta-Mendota, and Tracy Subbasins,respectively and on the northwest and north by the Solano,South American, and Cosumnes Subbasins. The Solano and South American are Subbasins of the Sacramento Valley Groundwater Basin. The Eastern San Joaquin Subbasin is drained by the San Joaquin River and several of its major tributaries namely,the Stanislaus, and Calaveras, and Mokelumne Rivers(Figure 2). The San Joaquin River flows northward into the Sacramento and San Joaquin Delta and discharges into the San Francisco Bay. Annual precipitation within the subbasin ranges from about i 1 inches in the southwest to about 25 inches in the northeast. Main\D�Environmental1%StE\Workplan%Workplan l l 192007.wpd 13 {{II II 1