and release water gradually through the dry season. The combined San Gabriel/Rio Hondo system is served by seven spreading grounds – San Gabriel Canyon, Santa Fe, Peck Road, San Gabriel Valley, Rio Hondo Coastal, San Gabriel Coastal and Montebello Forebay – totaling . The first three contribute to the San Gabriel Valley aquifer and recharge about each year. The others are used to recharge the Central Basin (coastal) aquifer and conserve an average of per year. In addition, rubber dams can be inflated along certain stretches of the San Gabriel River to slow the flow rate and allow water to percolate directly through the river bed. Water distribution in the San Gabriel Valley is adjudicated by the Main San Gabriel Basin Watermaster, a board which determines the amount of water to be delivered to each user (mostly municipal water agencies), recharged into the aquifer, and pumped from the aquifer. The "operating safe yield" is the amount of groundwater that can be reliably extracted from the aquifer and is determined by the Watermaster based on annual rainfall and runoff. Between 1973 and 2002 this averaged approximately . The Central Basin Watermaster serves the same purpose for the Central Basin aquifer and allows pumping of roughly per year. The Puente Subbasin is located between the Puente and San Jose Hills (roughly between City of Industry and Diamond Bar) and although it is hydrologically part of the San Gabriel Valley aquifer, is managed as a separate entity. Hydroelectricity There is one hydroelectric plant on the river, located just to the north of Azusa. The original Azusa Hydroelectric Plant was built in 1898 by the San Gabriel Electric Company (which in 1917 was incorporated into Southern California Edison). Power generation began on June 30, with an initial capacity of 2,000 kilowatts (KW). During the early 1900s it was mainly used to power the Pacific Electric (Red Car) and Los Angeles Railway (Yellow Car) systems in the greater Los Angeles area. The plant was purchased by the City of Pasadena in 1930, due to structural modifications needed to accommodate the city's proposed Morris Dam. A new 3,000 KW plant was built adjacent to the old plant in the 1940s. The power station is supplied with water via the long Azusa Conduit, which draws water from the river below San Gabriel Dam, and runs along the east wall of the San Gabriel Canyon to a point