Chatsworth Reservoir was placed in service in 1919 when the San Fernando Valley was just a agricultural area. The reservoir has two earthfill dams which provided an initial capacity of 7,400 acre-feet and a high water elevation of 884.3 feet. The dams were built using the process of hydraulic-fill which means that the soil material forming the dam was washed or sluiced into place, and no mechanical compaction equipment was used to densify the material. Because this material has little or no compaction, it has low density and low strength or ability to resist earthquake loading.
From 1920 to 1950 Chatsworth Reservoir served as the principal water storage facility in the western San Fernando Valley, serving the irrigation needs of the area’s farming community. The reservoir had improvements done in 1930 and 1931 to increase the high water level of the reservoir 5 feet to elevation 889.3 feet and the capacity to 9,840 acre-feet.
The transition from strictly agriculture to both agriculture and residential/domestic drinking water service resulted in a problem of maintaining water quality. In the summer, the Chatsworth Reservoir would form a number of shallow bays and in warm weather they would encouraged the growth of algae and aquatic plants. During winter storm water runoff would flow into the reservoir and along with stormy winds would cause wave action stirring up mud in the shallow areas of the reservoir. Contaminated surface water storm flow and groundwater from surrounding aerospace facilities were showing up in reservoir wells. These problems degraded the water quality and caused the reservoir to be removed from service each summer.
In 1971, the Sylmar earthquake severely damaged the dams, and because of the damage, the State Department of Water Resources (DWR), Division of Safety of Dams, directed a thorough stability investigation of the dams. The study showed that the Chatsworth Dams would not perform in a satisfactory manner if subjected to another major earthquake, either a local or San Andreas event. As a result of this it was determined that the two dams forming Chatsworth Reservoir would have to be completely rebuilt, and that no water could be stored in Chatsworth Reservoir.