San Gabriel River and Montebello Forebay Water Conservation System
ogswell Dam and San Gabriel Dam in the San Gabriel Canyon are owned by the Flood Control District and operated by Public Works. A third dam, Morris Dam, is located a few miles below San Gabriel Dam, was built by the City of Pasadena and later relinquished to the Metropolitan Water District (MWD) of Southern California. In 1995, Morris Dam was transferred to the Department of Public Works.
Work on Cogswell Dam commenced in 1932 and was completed in April 1934 at a cost of $3.1 million. It is a rock-filled structure with a concrete cutoff wall and rises 255 feet above the original stream bottom.
San Gabriel Dam construction began in 1932 and was completed in 1939 at a cost of $17 million. It is a compacted earth-fill and rock-fill dam with a concrete cutoff wall standing 310 feet above the original streambed. In the late 1938's, the spillway was completed and the penstocks and valves were installed. Just downstream of the outlet valve is a water measuring structure and filtering box or "sand box" for sands to settle out. Hydroelectric generation facilities have been installed just upstream of this structure. The "sand box" receives controlled flows from the dam and is the beginning of the Azusa Conduit, and eight-mile tunnel leading to the facilities of downstream water users. At the end of the tunnel are two penstocks, one of which directs flows to a hydroelectric power station owned by the City of Pasadena.
Morris Dam was completed in May 1934. It is a concrete, partially arched gravity structure rising 245 feet above the original streambed and is located a few miles below San Gabriel Dam.
In addition to releases from these canyon dams, flows through the mouth of the canyon may originate from a MWD outlet, Upper San Gabriel No. 3 located below Morris Dam, which can deliver untreated imported water to downstream spreading facilities to augment local water replenishment. The San Gabriel Valley Municipal Water District also delivers imported water to the San Gabriel Canyon Spreading Grounds and to the San Gabriel River for spreading.
Located at the mouth of the canyon is the San Gabriel Canyon Spreading Grounds, which has been used for spreading canyon water since 1914. In recent years, this area has been excavated into two large basins.