U.S. Geological
Survey U.S. Geological
Survey 
The Santa Clara Valley is underlain by large amounts of clay that readily compacts as a result of excessive ground-water withdrawal, thus causing land subsidence. Land subsidence has been evident over much of the valley and is greater than 8 feet in some places (fig. 115). Subsidence has resulted in flooding in coastal areas and damage to roads, bridges, railroads, and sewer systems. The cost of remedial measures has been estimated to be between $30 million and $50 million annually. The rate of subsidence slowed in 1967 as increased surface-water imports and reduced ground-water withdrawals allowed the hydraulic head to stabilize and start to recover. Under current (1995) conditions of ground-water use and availability, further subsidence is not likely. However, because the compression of the clay is irreversible, land subsidence that has already occurred is permanent.