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<br />TIll' state '.'I enormous <br />grol/l1du'atel' aquifers <br />arc estimated to hold <br />850 million acre-feet <br />(~rH'atcr - 20 times the <br />al1loul1f (~f\l'ater that eWI <br />be stored behind all <br />(~{ Cal{{omia 's dams <br /> <br /> <br />WATER BEARING FORMATIONS <br /> <br /> <br />Volcanics: <br />(dark gray) <br /> <br />Rocks, such as in Modoc Plateau in <br />northeastern Calitornia, Santa Monica <br />Mountains, Napa Valley and other <br />areas of California. Yields of wells <br />vary considerably from high to low. <br /> <br />Alluvium: <br />(light gray) <br /> <br />Gravel, sand, silt and clay all deposited <br />by streams and lakes in the last 3 <br />million years. Wells in these basins <br />usually have high yieids. <br /> <br />Fractured <br />Hard Rock: <br /> <br />Mountain-forming rock such as in the <br />Sierra Nevada, Coast Ranges and <br />Klamath Mountains. Wells in these <br />areas generally supply the lowest <br />water yield. <br /> <br />3 <br />