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The 'Whittier Narrows' is located at the southern boundary of the San Gabriel Valley, in Los Angeles County, California. It is a gap in the Puente Hills where the Rio Hondo and the San Gabriel River converge.
Whittier Narrows Dam is a flood control and water conservation project constructed and operated by the U.S. Army Corps of Engineers, Los Angeles District. Construction of the project was completed in 1957. The project is located, as its name implies, at the "Whittier Narrows". The Whittier Narrows are a natural gap in the hills that form the southern boundary of the San Gabriel Valley. The Rio Hondo and the San Gabriel River flow through this gap and are impounded by the reservoir. The Pomona (60) freeway passes through the reservoir flood control basin and the San Gabriel (605) freeway passes along the eastern boundary of the basin. Authorization for the project construction is contained in the Flood Control Act of 18 August 1941 (PL 77-228) and the initial funds for construction were provided in the 1949 Appropriations Bill.
Whittier Narrows Dam provides water conservation storage and is also the central element of the Los Angeles County Drainage Area (LACDA) flood control system. The purpose of the project is to collect runoff from the uncontrolled drainage areas upstream along with releases into the San Gabriel River from Santa Fe Dam. If the inflow to the reservoir exceeds the groundwater recharge capacity of the spreading grounds along the Rio Hondo or the bed of the San Gabriel River downstream, this water is stored temporarily in a water conservation pool. The Rio Hondo and San Gabriel sides of the reservoir each have their own water conservation pools. If the water conservation pool on the Rio Hondo side is exceeded, flows are released into the Rio Hondo at a rate which not exceed the downstream channel capacity of either the Rio Hondo, or the Los Angeles River. If the water conservation pool on either side of the reservoir is exceeded a release of approximately 5000 cfs (142 cms) can be made into the San Gabriel River. If the pool in the reservoir exceeds flood control storage, the gates on the San Gabriel River outlet begin to open automatically and emergency releases are made into the San Gabriel River.
The Rio Hondo outlet has has four main outlet passages plus a small diversion passage. The San Gabriel outlet has nine large gates installed on top of a spillway. Dimensions are furnished in the table below.
The "stand-by" position of the gates on the Rio Hondo outlet is wide open. On the San Gabriel side one gate is normally open about 0.5 feet (0.15 meters) with the remaining gates closed. The reservoir is normally empty and a "crossover weir" within the reservoir keeps the flows from the Rio Hondo and the San Gabriel River separated. The natural flow to each river therefore normally passes through the dam unhindered.
During the initial stages of a flood event, the gates on the Rio Hondo side are partially closed to build a water conservation pool. As long as the pool on the Rio Hondo side of the reservoir is below elevation 201.6 feet (61.4 meters) NGVD, releases are made to accommodate the capacity of the spreading grounds downstream along the Rio Hondo. The spreading grounds are operated by the Los Angeles County Department of Public Works to recharge the groundwater basin. Flow reaches the spreading grounds either directly by way of the diversion passage or from a diversion structure in the Rio Hondo downstream of the dam. Both the diversion passage direct from the dam and the diversion structure in the Rio Hondo are operated by the county.
When the water conservation pool on the Rio Hondo side of the reservoir is exceeded, the releases to the Rio Hondo are increased to match inflow until either the capacity of the Rio Hondo or the Los Angeles River downstream are reached. If the water conservation pool on either side of the reservoir is exceeded, discharges on the San Gabriel side can be increased to approximately 5000 cfs (142 cms).
The San Gabriel outlet has automatic spillway gates. When the pool in the reservoir exceeds flood control storage these gates will begin to open automatically. The top of the flood control storage pool is at elevation 228.5 feet (69.6 meters) NGVD.
The capacity of the Rio Hondo downstream from Whittier Narrows Dam is approximately 36,500 cfs (1,034 cms). The capacity of the Los Angeles River downstream of its confluence with the Rio Hondo is approximately 127,000 cfs (3,596 cms), and the capacity of the San Gabriel River downstream of the dam is approximately 13,100 cfs(371 cms).
The Whittier Narrows earthquake, which measured 5.9 on the Richter Scale, occurred October 1, 1987.

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★ Whittier Narrows Dam