contributing greatly to irrigation in the Sacramento Valley and navigation on the Sacramento River, as well as keeping freshwater levels in the Sacramento-San Joaquin Delta high enough for diversion into the California Aqueduct and Delta-Mendota Canal. The dam's other major purpose is to generate hydroelectricity. With a hydraulic head of , the dam is capable of generating 676 megawatts (MW) from five turbines – a pair of 125 MW units and three 142 MW units. Each of the turbines is driven by a high-pressure jet of water fed by a steel penstock in diameter. Two smaller turbines generate power for operations at the dam itself. The plant serves to generate peaking power for the northern Sacramento Valley. Keswick Dam, about downstream, serves as an afterbay for Shasta, regulating its fluctuating water releases. A gravity structure, the dam stands above the foundations with a maximum height of above the river. It is long, with a maximum thickness of ; altogether the dam contains of material. The dam can release floodwaters through a system of eighteen outlet valves on the face of the spillway. These valves are arranged in three levels, each cutting through the main dam structure and discharging onto the face of the spillway. The upper level has six outlets, each with a capacity of . The middle layer has eight conduits capable of carrying and the lowest has four exits each able to discharge for a total of . The spillway is a massive concrete chute, long and wide, controlled by three -wide drum gates each weighing 500 US tons (454 t). When the reservoir is full, the gates cannot entirely prevent leakage but can raise the water level up to above the spillway crest. The spillway has a capacity of , bringing the dam's maximum overflow rate to . The dam forms a reservoir called Shasta Lake, which is the largest man-made lake and third largest body of water in California with its capacity of and surface area of at maximum pool. The lake extends for up the Sacramento River and branches for more than up the Pit River, which is actually the largest river flowing into the lake. Shasta Lake also has arms of the McCloud River, Squaw Creek, Salt Creek, and scores of other smaller streams that feed it. Shasta Dam controls runoff from a drainage basin of , or about a quarter of the Sacramento