The San Gabriel River water conservation system begins in the San Gabriel Mountains with the capture of storm runoff and snow melt in the reservoirs of Cogswell, San Gabriel, and Morris Dams. Water released through valves or passing over spillways when the reservoirs are full can be diverted at the mouth of the canyon to the San Gabriel Canyon Spreading Grounds or can continue downstream in the unlined San Gabriel River toward the U.S. Army Corps of Engineers' (Corps) Santa Fe Dam. In the upper portion of the Santa Fe Reservoir is the Santa Fe Reservoir Spreading Grounds. Releases from Santa Fe Dam can be spread in the unlined San Gabriel River downstream with large flows continuing to Whittier Narrows Dam. Water can also be diverted just below Santa Fe Dam and routed via Sawpit Wash to Peck Road Spreading Basin and the beginning of the Rio Hondo water conservation system (not shown here).
The Corps' Whittier Narrows Dam is at the northern boundary of the Montebello Forebay. San Gabriel River flows arriving in the reservoir can be directed either to the San Gabriel Coastal Basin Spreading Grounds and the downstream unlined San Gabriel River, or to the Rio Hondo Coastal Basin Spreading Grounds. Any water released from Whittier Narrows Dam that is not captured in the Coastal Plain spreading facilities flows on to the ocean.