Whittier Narrows Dam
| Whittier Narrows Dam | |
|---|---|
Aerial view of Whittier Narrows Dam looking upstream through the Rio Hondo Spillway. | |
| Location | Los Angeles County, California, United States |
| Coordinates | 34°01′12″N 118°04′58″W / 34.02000°N 118.08278°W |
| Opening date | 1956 |
| Dam and spillways | |
| Impounds | Rio Hondo San Gabriel River |
| Height (foundation) | 56 feet (17 m) |
| Length | 16,960 feet (5,170 m) |
| Elevation at crest | 236 feet (72 m) |
| Width (crest) | 32 feet (9.8 m) |
| Dam volume | 12,166 cu yd (9,302 m3) |
| Spillway type | Automated spillway overflow |
| Parapet width | 16 ft (4.9 m) |
| Hydraulic head | 182 ft (55 m) |
Whittier Narrows Dam is a 56-foot (17 m) tall earth dam on the San Gabriel River and the smaller, parallel Rio Hondo. The dam is located, as the name implies, at the Whittier Narrows. It provides water conservation storage and is also the central element of the Los Angeles County Drainage Area (LACDA) flood control system. Its reservoir has a capacity of 67,060 acre⋅ft (82,720,000 m3).
The Whittier Narrows are a natural gap in the hills that form the southern boundary of the San Gabriel Valley of Southern California. Both the Rio Hondo, a tributary of the Los Angeles River, and the San Gabriel River flow through this gap and are impounded by the reservoir. The Pomona Freeway (CA-60) passes through the reservoir flood control basin and the San Gabriel River Freeway (I-605) passes along the eastern boundary of the basin.
In September 2017, the United States Army Corps of Engineers officials warned local residents that the dam no longer met the agency’s 'tolerable-risk' guidelines and could fail in the event of a very large, very rare storm, similar to exceptionally intense California storms which occurred between December 1861 and January 1862, a so-called ARkStorm
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. The U.S. Army Corps of Engineers completed construction of the dam in 1957.