Mammoth Pool Dam
| Mammoth Pool Dam | |
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 Location of Mammoth Pool Dam in California
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| Country | United States |
| Location | Fresno / Madera counties, California |
| Coordinates | 37°19′23″N 119°18′58″W / 37.32306°N 119.31611°WCoordinates: 37°19′23″N 119°18′58″W / 37.32306°N 119.31611°W |
| Construction began | 1958 |
| Opening date | 1959 |
| Dam and spillways | |
| Type of dam | Rockfill |
| Impounds | San Joaquin River |
| Height | 411 ft (125 m) |
| Length | 820 ft (250 m) |
| Width (crest) | 30 ft (9.1 m) |
| Width (base) | 2,100 ft (640 m) |
| Dam volume | 5,250,000 cu yd (4,010,000 m3) |
| Spillways | Ungated overflow |
| Spillway capacity | 170,000 cu ft/s (4,800 m3/s) |
| Reservoir | |
| Creates | Mammoth Pool Reservoir |
| Total capacity | 123,000 acre·ft (0.152 km3) |
| Catchment area | 998 sq mi (2,580 km2) |
| Surface area | 1,100 acres (450 ha) |
| Normal elevation | 3,337 ft (1,017 m) (max) |
| Power station | |
| Operator(s) | SCE |
| Hydraulic head | 1,004 ft (306 m) |
| Turbines | 2x 95 MW |
| Installed capacity | 190 MW |
| Annual generation | 580.3 million KWh (1990-2004) |
Mammoth Pool Dam is a hydroelectric dam located on the San Joaquin River in the southern Sierra Nevada mountain range of California, about 45 miles (72 km) northeast of Fresno. It forms Mammoth Pool Reservoir and lies within the Sierra National Forest. The dam and reservoir were named after a large natural pool in the river that was once located above the present dam site.
In 1900, engineer John S. Eastwood and several wealthy investors founded the Mammoth Power Company with the goal of harnessing hydroelectric power from the upper San Joaquin River. The Mammoth Pool site, where the San Joaquin River canyon chokes down to a narrow gap between massive walls of solid granite, was immediately seen as an attractive dam site. The company's first proposal was to construct a rockfill diversion dam that would send water into a 20-mile (32 km)-long tunnel, developing a hydraulic head of over 1,700 feet (520 m) to supply a 120,000 hp powerhouse. However, the investors involved with Mammoth eventually lost interest due to the enormous scope of the project. By 1901, the Mammoth Pool proposal was shelved.
During the mid-1950s, development of hydroelectric facilities in the upper San Joaquin basin began to pick up again and plans were finalized for the construction of Mammoth Pool and its associated power station. Construction of the main dam embankment started in October 1958 after the San Joaquin River was diverted through a 2,150-foot (660 m)-long tunnel to bypass the dam site. Work progressed fairly quickly, although the builders ran into problems with exfoliation of granite sheets, probably caused by drilling and explosive charges along the steep walls of the gorge. The dam was topped out almost exactly one year later in late 1959.
The dam serves mainly for the generation of hydroelectric power. It is an important component of Southern California Edison's Big Creek Hydroelectric Project, which comprises a system of 25 dams, nine power plants and supporting tunnels and diversion channels in the upper basin of the San Joaquin River. Water from the reservoir is diverted into a 7.45-mile (11.99 km)-long reinforced concrete into Mammoth Pool Powerhouse, which has two 100,000 hp turbines totaling a nominal generating capacity of 190 megawatts (MW). Due to friction decreasing flow rate in the power tunnel, the actual output during maximum generation is only about 187 MW. A 350 ft (110 m) high surge chamber is located shortly above the powerhouse to provide protection to the turbines in case of abrupt flow fluctuations through the tunnel. The 1,004 ft (306 m) hydraulic head afforded to the power plant is almost three times greater than the dam's structural height.
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