The Santa Clara Valley is located at the southern end of San Francisco Bay (fig. 111). Once devoted largely to agriculture, most of the land in the valley is now dedicated to industrial and urban uses. Population growth has resulted in a large water demand, which has exceeded the valley's natural supply since the early 1940's. Withdrawals of ground water in excess of recharge caused large water-level declines, which were followed by seawater intrusion and land subsidence surpassed in California only by that in the San Joaquin Valley. Since the 1940's, importation of surface water has been essential to the control of these problems.
The Santa Clara Valley is in a structural trough that parallels the northwest-trending Coast Ranges. The drainage basin, which includes San Francisco Bay, is bounded by the Santa Cruz Mountains on the southwest and the Diablo Range on the northeast. The basin is about 75 miles long and has a maximum width of 45 miles. The San Andreas Fault is in the Santa Cruz Mountains to the southwest, and the Hayward Fault is on the northeast side of the valley and parallels the Diablo Range (fig. 112). The Santa Clara Valley, which occupies the southern end of the basin, is about 60 miles long, about 30 miles of which extends southeastward beyond San Francisco Bay. The valley has a maximum width of about 15 miles and a total area of about 590 square miles. The altitude of the valley floor ranges from about 350 feet at the southern end to sea level at San Francisco Bay.
The Mediterranean climate of the valley is moderate and has distinct wet and dry seasons. The wet season extends from November to April. Average annual rainfall is about 14 inches on the valley floor.