Shasta Dam - Design and Operations

Design and Operations

Shasta Dam serves mainly to provide flood control and carryover water storage for the dry season, 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 330 feet (100 m), 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 15 feet (4.6 m) 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 9 miles (14 km) downstream, serves as an afterbay for Shasta, regulating its fluctuating water releases.

A thick gravity-arch structure, the dam stands 602 feet (183 m) above the foundations with a maximum height of 522.5 feet (159.3 m) above the river. It is 3,460 feet (1,050 m) long, with a maximum thickness of 543 feet (166 m); altogether the dam contains 6,270,000 cubic yards (4,790,000 m3) 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 6,534 cubic feet per second (185.0 m3/s). The middle layer has eight conduits capable of carrying 3,100 cubic feet per second (88 m3/s) and the lowest has four exits each able to discharge 4,450 cubic feet per second (126 m3/s) for a total of 81,800 cubic feet per second (2,320 m3/s). The spillway is a massive concrete chute, 487 feet (148 m) long and 375 feet (114 m) wide, controlled by three 110-foot (34 m)-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 28 feet (8.5 m) above the spillway crest. The spillway has a capacity of 186,000 cubic feet per second (5,300 m3/s), bringing the dam's maximum overflow rate to 267,800 cubic feet per second (7,580 m3/s).

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 4,552,000 acre feet (5,615,000 dam³) and surface area of 29,740 acres (12,040 ha) at maximum pool. The lake extends for 15.3 miles (24.6 km) up the Sacramento River and branches for more than 21 miles (34 km) 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 6,665 square miles (17,260 km2), or about a quarter of the 27,580-square-mile (71,400 km2) Sacramento River watershed.