The Anza-Terwilliger area consists of about 96 square miles (24-9 square kilometers) in the upper parts of the Santa Margarita River and Coyote Creek drainage basins in Riverside County, Calif., about 90 miles (145 kilometers) southeast of Los Angeles. This report deals with geology, steady state and transient state of ground water, net depletion of ground water, surface-water flow, precipitation, chemistry of water, land and water use, and gravity data for the Anza-Terwilliger area. The data indicate that the rate of ground-water depletion ha's accelerated since 1950. Pumping depressions adjacent to the Cahuilla Indian Reservation have increased the hydraulic gradient and are.causing water beneath the reservation to flow toward these depressions. Total depletion of ground water since 1950 is about 14,000 acre-feet (17.3 cubic hectometers). Chemical analyses indicate that the ground water in local areas contains concentrations of nitrate above that recommended by the Environmental Protection Agency for human consumption.