just north of Azusa adjacent to the San Gabriel Canyon spreading grounds, where a diameter penstock falls down the mountainside to the powerhouse. Between 1996 and 2014 the plant generated an annual average of 4 million kilowatt hours. The usage of river water for electricity production has been controversial, as diverting water can dry up the channel, reducing fish habitat. Sand and gravel mining Although not directly related to water supply, the San Gabriel River bed –filled with coarse and fine sediments to depths of hundreds and sometimes thousands of feet – is an important source of aggregate materials (gravel and sand) for use in construction. The San Gabriel Valley around Irwindale is one of the largest aggregate mining areas in the United States – more than a billion tons have been taken from the old river bed, supplying construction projects all over Los Angeles County. Most of the freeway system in greater Los Angeles was built using aggregate from the San Gabriel river bed. In Irwindale there are seventeen gravel pits of various sizes, although not all are being mined. The largest aggregate company operating in the San Gabriel river is Vulcan Materials Company. There are proposals to refill some of the inactive pits to allow commercial, retail and industrial development, or repurpose them as parks or water storage reservoirs. The maximum allowed depth is , and since many pits have already reached this depth, mining companies are pushing to extend the limit by another . This has been controversial due to the risk of slope instability. Another major area of sediment removal is from the reservoirs along the San Gabriel River. The San Gabriel River drains one of the most erosive mountain ranges in the world, and mountain reservoirs must be continually dredged to maintain enough space for flood control. Between 1935 and 2013 about of sediment have been removed from Cogswell and San Gabriel Reservoirs, equal to about 40 percent of the total original design volume of the reservoirs. Most of this material is unsuitable for use as aggregate and must be disposed of in designated sediment placement sites. It has been proposed to truck reservoir mud to Irwindale to fill some of the abandoned gravel quarries there. Water quality issues Since more than half the watershed is developed, the San Gabriel River receives large amounts of industrial and urban runoff that contribute to pollution in the