San Antonio Dam is a flood control and water conservation project constructed and operated by the U.S. Army Corps of Engineers, Los Angeles District. The San Antonio Flood Control Project including the San Antonio and Chino Creek Channels Improvements Project was authorized (as part of the Santa Ana River Basin flood protection program) by the Flood Control Act of June 22, 1936 (PL 74-738) and the Flood Control Act of June 28, 1938 (PL 75-761). The contruction of the dam was initiated in April 1952 and completed on May 1, 1956. The construction of the San Antonio and Chino Creek Channels was initiated in 1956 and completed in 1960.
San Antonio Flood Control Project is located approximately 30 miles east of Los Angeles in the Santa Ana River Basin. The dam is situated on San Antonio Creek about 10.5 miles upstream from its confluence with Chino Creek, which is a tributary to the Santa Ana River. San Antonio Creek originates in the San Gabriel Mountains on the south slopes of San Antonio Peak at elevation 10,064 feet NGVD. It flows in a southerly direction appproximately 11 miles into San Antonio Reservoir, draining an area of 26.7 square miles. The San Antonio Reservoir lies mostly in San Bernardino County with only a small portion falling within the Los Angeles County boundary line. The dam is sited at the mouth of the canyon where San Antonio Creek emerges from the San Gabriel Mountains.
The gate "standby" position is one gate open at 0.3 feet with the remaining two gates closed. During the initial stages of an inflow event, the 3 gates remain at the "standby" setting to form a debris pool until the water surface elevation rises above elevation 2,164 feet NGVD. From water surface elevation 2,164 feet NGVD to 2,170 feet NGVD, outlet gates are raised to increase releases from 80 cfs to 5,030 cfs. When water surface elevation rises above 2,170 feet NGVD, an average release of 7,500 cfs is maintained with the maximum release capped at 8,000 cfs. When the water surface rises to elevation 2,238 feet NGVD, uncontrolled spillway flows will begin. During the initial spillway flows, releases from the outlet gates are adjusted so that the combined spillway flow and the outlet gates outflow will not exceed 8,000 cfs. When the uncontrolled releases exceed 8,000 cfs, the controlled releases from the outlets are shut off. Flood releases from San Antonio Dam plus local downstream runoff are discharged into the Prado Reservoir, another project operated by the Corps in the Santa Ana River Basin downstream from the San Antonio Dam.
During the falling stages the operational schedule is followed in reverse until the water surface level falls to elevation 2,176 feet NGVD. When the water surface elevation falls below 2,176, a decision can be made to continue flood control releases or go off-schedule for water conservation operation. During the water conservation operations, releases from San Antonio Dam are coordinated with the city of Pomona, Water Operations Division. Most of the releases, if not all, coming out of San Antonio Dam is diverted for spreading operations by the city of Pomona as part of the Pomona Valley Protective Association (PVPA). Any excessive flow going down the San Antonio Creek can be diverted by the Mountain View Water Company, the Chino Baisn Municipal Water District, and the Chino Basin Water Conservation District.
The 15.7-mile San Antonio and Chino Creeks channel improvement project provides the following channel capacities: (1) channel capacity is 8,000 cfs immediately downstream of the the dam; (2) channel capacity increases from 8,000 cfs at the dam to 17,000 cfs at the Chino Creek confluence; and (3) channel capacity increases from 17,000 cfs at the Chino Creek confluence to 29,000 cfs at the discharge point to the Prado Reservoir. Diversions for water conservation were provided in the improved channel for the Pomona Valley Protective Association (PVPA), the Mountain View Water Company, and the San Bernardino Flood Control District.
The current water control manual for San Antonio Dam was approved in July 1991.