LEARN ABOUT
San Gabriel River and Montebello Forebay Water Conservation
BACKGROUND
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.
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.
SYSTEM PROFILE
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.
CANYON FACILITIES
Cogswell 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.
DOWNSTREAM FACILITIES
When San Gabriel River water is released from the canyon facilities, it flows to the Santa Fe Dam, a compacted earth-fill dam owned by the Corps, rising 92 feet in height and spanning 23,800 feet in length at the crest. In the upper portion of the reservoir is the Santa Fe Spreading Grounds. This excellent spreading facility, which Public Works operates under license from the Corps, can percolate approximately 400 cubic feet per second continuously. A rubber dam, built in 1995, is used to direct flows from the San Gabriel River into the Santa Fe Spreading Grounds. The San Gabriel River downstream of Santa Fe Dam was constructed as a soft bottom channel to promote infiltration of water released from the dam during large storms or for spreading.
A rubber dam, built in 1994, is located in the San Gabriel River near Valley Boulevard. This rubber dam creates a 76-acre, 495 acre-foot spreading facility within the San Gabriel River and Walnut Wash.
The Whittier Narrows Dam, also owned by the Corps, captures water flowing in the San Gabriel River and Rio Hondo Channel. Whenever the primary objective of flood control is not compromised, controlled released of water from Whittier Narrows Dam continue to Public Works’ Coastal Plain spreading grounds
In addition to spreading storm water, MWD provides imported water to Public Works for groundwater recharge in cooperation with the Main San Gabriel Basin Watermaster and the Central and West Basin Municipal Water Districts. Pictured below is Central Basin Connection No. 48, a MWD outlet that can release water to San Dimas Wash and then through the downstream flood control system to the Montebello Forebay. Another agency that provides imported State Water Project water for spreading is the San Gabriel Valley Municipal Water District, which has its own delivery pipeline. About 75,000 acre-feet of untreated imported water purchased by local water agencies are spread annually.
A further aspect of the water conservation program is spreading recycled water from facilities such as Whittier Narrows, San Jose Creek, and Pomona Water Reclamation Plants owned by the Sanitation Districts of Los Angeles County. Planned replenishment using recycled water has been practiced in the Montebello Forebay area since 1962. Currently, about 50,000 acre-feet of recycled water is spread each year on behalf of the Water Replenishment District of Southern California. Plans are under way to spread 10,000 acre-feet of recycled water in the San Gabriel River below Santa Fe Dam on behalf of the Upper and San Gabriel Valley Municipal Water Districts.
MONTEBELLO FOREBAY FACILITIES
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 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.
ONGOING IMPROVEMENT
Since its inception in 1915, the Flood Control District, now Public Works, has made great strides in conserving water for Los Angeles County along the San Gabriel River, in the Montebello Forebay, and elsewhere. But storm runoff has increased with urbanization and more can be accomplished through extensive planning and improvement programs.
It is estimated that the San Gabriel Canyon watershed generates about 1.3 million cubic yards (mcy) of sediment annually. Since construction of the Cogswell, San Gabriel, and Morris Dams in the 1930’s, approximately 68 mcy of sediment has deposited in the three dams. Over the years, the Department has removed about 36 mcy, while 32 mcy of sediment remains trapped behind the dams. The accumulated sediment has resulted in a loss of flood control and water conservation capabilities at these dams. A sediment management plan has been developed that identifies methods and costs to remove the sediment so that water storage capacity can be maintained in the future.
Groundwater recharge improvements recently made or pending along the San Gabriel River include construction of intake structures to the San Gabriel Canyon Spreading Grounds, improvement of Santa Fe Reservoir Spreading Grounds, acquisition of abandoned gravel pits for spreading in the San Gabriel Valley, and increasing the spreading of reclaimed water.
The Montebello Forebay spreading facilities have recently undergone improvements with some work remaining including automation and telemetry facilities.
Long-ranged planning for Public Works’s water conservation facilities will include consideration of improving aesthetics and providing for more recreation. However, the primary goal of the water conservation will continue to be maximizing water conservation capabilities to maintain the County’s vital supply of groundwater.
FACILITIES
