San Gabriel River and Montebello Forebay Water Conservation System


the Montebello Forebay, located just south of Whittier Narrows, is a valuable area for groundwater recharge due to its highly permeable soils which allow deep percolation of surface waters. Located within the Forebay are the Rio Hondo Coastal Basin Spreading Grounds, the San Gabriel Coastal BasinRio Hondo Coastal Basin Spreading Grounds, and the Lower San Gabriel River spreading area. Current operations at these recharge facilities conserve an average of approximately 150,000 acre-feet of local, imported, and reclaimed water annually. Supporting the operations at these facilities is a water quality monitoring program.

The Rio Hondo Coastal Basin Spreading Grounds, Public Works' largest spreading facility, cover about 570 acres. Water is diverted from the Rio Hondo Channel by use of three large radial gates. The grounds have recently undergone major changes to increase their capacity. Fifty-two shallow (4 to 6 feet deep) basins have been excavated to form 20 larger and deeper (6 to 10 feet) basins, a new intake structure has been constructed which more than double the prior intake capacity, and new control and water delivery structures have been installed. The storage capacity has been almost tripled. These improvements have resulted in the capture of greater amounts of storm flows.

The San Gabriel River Coastal Basin Spreading Grounds, totaling 128 acres in size, have also been improved. Three deeper and larger spreading basins have been formed where 11 existed. This has more than doubled the storage capacity of the facility. Located at the headworks of the spreading grounds is an inflatable rubber dam used to divert flows to the grounds or regulate releases downstream.

The lower San Gabriel River, from Whittier Narrows Dam to Florence Avenue, also allows spreading by percolation through its unlined bottom. Five inflatable rubber dams were installed in the 1980's to increase spreading capacity along this portion of the river, replacing sand levees that washed out when high flows occurred.

San Gabriel Coastal Basin Spreading Grounds