Water enters the aquifer system at the valley margins by infiltration from the small streams that emanate from the mountains and by rainfall that falls directly on the valley floor. The natural, or predevelopment, flow pattern was generally parallel to the direction of stream drainage, and water that did not leave the aquifer system by way of evapotranspiration discharged into San Francisco Bay (fig. 112). The Hayward Fault acts as a major impediment to flow on the northeastern side of the valley.
In 1915, the hydraulic head was above land surface throughout much of the valley, and flowing wells were common. However, by 1967, an increase in ground-water withdrawals, as well as below-normal rainfall, resulted in water-level declines of more than 200 feet below 1915 levels in some parts of the valley (fig. 113). Large withdrawals lowered water levels to below sea level over much of the valley and reversed the freshwater gradient in the confined zone from seaward to landward. This reversal resulted in seawater intrusion that was detected in wells as far as 10 miles inland. The large withdrawals also caused widespread land subsidence.
Beginning in the mid-1960's, the decline in artesian head was halted and reversed by a combination of surface-water imports and decreased ground-water withdrawals (fig. 114). Water importation into the Santa Clara Valley began in about 1940 by way of the Hetch Hetchy Aqueduct and was increased in the mid-1960's through the South Bay Aqueduct. By 1980, surface-water imports approximately equaled ground-water withdrawals. Projections of future water demand made in 1983, however, indicated that by 2000 small amounts of additional surface water will have to be imported and the distribution system improved.