San Gabriel River and Montebello Forebay Water Conservation System


For centuries, flood water have periodically swept out of the San Gabriel Mountains causing extensive damage and sometimes taking a great toll of lives. The disastrous flood of 1914, which caused over $10 million in property damage, prompted the creation of the Los Angeles County Flood Control District. The District was formed to control and conserve flood, storms, and other waste waters. In 1985, District personnel were merged with other County engineering departments to form the Los Angeles County Department of Public Works. The duties of the district are now performed under contract by Public Works.

February 1914 Flood Successful bond issues in 1917 and 1924 financed construction of 14 dams which were built to impound San Gabriel Mountain storm waters until they could be released in a controlled manner. Two of these dams, now known as Cogswell and San Gabriel, were built in San Gabriel Canyon in 1934 and 1939, respectively, and a third dam, known as Morris Dam was constructed in 1934 by the City of Pasadena. During the large 1938 flood, Cogswell, San Gabriel, and Morris Dams were very beneficial in controlling what was the largest recorded flood up to that time even though the valves at San Gabriel Dam had not been installed. The dams then and since have prevented catastrophic damage to downstream areas such as that in the 1914 flood.

Public Works artificially recharges the groundwater supply aquifers underlying the County by spreading imported water, local runoff (including the water impounded by the upstream dams during storms), and recycled water at their 27 spreading facilities. The spreading facilities are located throughout the County along the main water courses and some of their tributaries. Imported and recycled water discharged into these channels and runoff resulting from storm events are diverted into the spreading facilities and allowed to percolate down to the water table. The water can then be pumped up to the surface for water supply purposes.

Public Works' major spreading facilities are located downstream of the San Gabriel Canyon and in the Montebello Forebay area. These facilities are the most significant in the County since operations at these facilities contribute the majority of water artificially recharged to the County's various aquifers.

Image of Cogswell Dam