Pinto basin, in the north-central part of Riverside County, Calif., is a typical desert valley formed by downfaulting along several major fault zones. The valley is filled with alluvium, and ground water in the alluvium discharges as subsurface outflow through an alluvium-filled gap at the east end of the valley. Occasionally surface water from cloudburst floods also discharges in a wash through the gap at the east end of the valley.
A northeastward extension of the major fault along the south side of the valley acts as a barrier to the discharge of ground water from the valley. The average ground-water gradient is less than 1 foot per mile across the main part of the valley above this barrier, but the water level drops abruptly across the fault. The ground-water storage capacity of the uppermost 100 feet of saturated alluvium beneath the central valley area is estimated to be about 230,000 acre-feet. All this water in storage occurs at depths greater than 95 feet below the land surface and cannot be reached by plants or animals. During 1959 virtually all the water pumped in the area was withdrawn from storage. However, the quantity of water pumped is small in relation to the total quantity in storage. Except for a small decline in head, no evidence indicates that the pumping will greatly impair the yield for many years or cause the water to deteriorate in quality.