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areas while clay,silt,and sand are dominant in the interfan areas. These units range in thickness from a thin veneer on the east side of the basin to over 150 feet near,the center of the basin. Groundwater occurs unconfined within these units. Well yields to 650± gpm are reported. Because these units are limited in thickness, most wells penetrate them in order to tap deeper aquifers in the area. Average specific yields in the 10-to 200-foot depth range vary from about 7 to 15 percent within the boundaries of the Tuolumne River Storage Unit(Davis and others, 1959). The average specific yield for fresh water bearing units in the San Joaquin County Groundwater Investigation area as defined in(DWR.1967) is 7.3 percent. The Victor Formation as defined in(DWR, 1967)is correlative with these units. Flood Basin Deposits. This unit-is exposed in the Delta area of the San Joaquin Valley. These deposits are basinward, fine-grained forms of the Laguna, Riverbank, Modesto, and Recent formations and, therefore, range in age from Pliocene to Recent. They are generally much finer grained with a higher percentage of fine sand and clays than their depositional equivalents to the east I and west. Occasional gravel beds occur along the present waterways and are probably representative of the type of underlying lithology distribution. This'unit ranges in thickness from 0 to 1,400 f feet. Groundwater in this,unit occurs under unconfined to confined conditions. The unit, in general, has low permeabilities and may create semi-confined to confined conditions when interfngered with the 1 Alluvium and Modesto/Riverbank Formations. Occasional pockets of fresh water are found in the Delta deposits, but generally speaking the formation contains poor quality water. This unit is designated as Dos Palos Alluvium by(Wagner and others, 1.991). I Laguna Formation. The Laguna Formation is Plio-Pleistocene in age and consists of discontinuous lenses of stream laid sand and silt with lesser amounts of clay and gravel. There are no regionally significant fine-grained intervals that could cause water pressure conditions, although the i heterogeneous nature of the sediments causes local confinement. From the Mokelumne River area. located near Lodi, the formation thickens from approximately 400 feet to approximately 1,000 feet in the.Stockton area. Regionally, yields of 1,500 gpm have been reported from highly permeable beds,but average yields are about 900 gpm.Groundwater occurs under unconfined to locally semi- ` En vironmentaMTETARSCRO 9102008.wpd 15 li