Whittier Narrows Dam

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 is 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 temporary 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.  Inflow into the reservoir from rainfall runoff can be temporarily stored in a water conservation pool and discharged at the groundwater recharge capacity of the spreading grounds along the Rio Hondo or the bed of the San Gabriel River downstream. The Rio Hondo and San Gabriel sides of the reservoir each have their own water conservation pools, although water conservation activities largely takes place on the Rio Hondo side of the project.  As the water levels in the reservoir rise above the maximum elevation of the water conservation pool regulation of the dam switches over from water conservation to flood risk management where discharge from the dam may be increased to maintain sufficient flood runoff storage behind the dam.  The current capacity immediately downstream of the dam is 40,000 cfs.   If the pool in the reservoir exceeds flood control storage (pool elevation greater than 228.5 ft, NGVD (69.6 meters), the gates on the San Gabriel River outlet begin to open automatically and uncontrolled spillway flow discharge will occur 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 to have three of the gates wide open and one gate set to an opening of 2.6 feet (0.79m). On the San Gabriel side one gate is normally open about 0.5 feet (0.15 meters) with all other gates being closed. The reservoir is normally empty, and when it receives rainfall runoff, two pools form, one at the Rio Hondo side and the other at the San Gabriel side, which are also separated by a bike path/weir.  This bike path/weir, also called the "cross-over weir" is exceeded when the pool elevation on the San Gabriel side exceeds 208 ft, NGVD, then flows over to the Rio Hondo side of the reservoir.  During normal drier times, the natural flow to each river normally passes through their respective outlets 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 in the outlet works or is diverted by a 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.

The capacity of the Rio Hondo downstream from Whittier Narrows Dam is approximately 40,000 cfs (1,133 cms). The  capacity of the Los Angeles River downstream of its confluence with the Rio Hondo is approximately 164,000 cfs (4,644 cms), and the capacity of the San Gabriel River downstream of the dam is approximately 13,100 cfs(371 cms).

The current approved water control manual for Whittier Narrows Dam is dated October 1957.