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.
Mammoth Pool is a rockfill structure standing 411 feet (125 m) above the foundations and 820 feet (250 m) long. The dam rises for 330 ft (100 m) above the bed of the San Joaquin River, and contains about 5,250,000 cu yd (4,010,000 m3) of material. The crest is situated at an elevation of 3,361 ft (1,024 m) and is approximately 30 ft (9.1 m) wide. The dam's base, by contrast, is more than 2,100 ft (640 m) thick.
The reservoir formed behind the dam has a conservation storage of 119,940 acre·ft (0.14794 km3), a maximum capacity of 123,000 acre·ft (0.152 km3), and can reach elevations of 3,337 ft (1,017 m) above sea level. Normal water releases from the dam are controlled by a cone (Howell-Bunger) valve with a capacity of 2,100 cu ft/s (59 m3/s) at maximum pool. Any higher water will flow over an uncontrolled ogee crest spillway cut into the rock of the San Joaquin Canyon about a quarter of a mile to the west of the main dam. The spillway is 403 ft (123 m) wide and has a capacity of approximately 170,000 cu ft/s (4,800 m3/s).
Mammoth Pool Dam has caused the submersion of a migratory route commonly used by mule deer in the Sierra, forcing them to swim across the reservoir during their spring and autumn migrations. The reservoir is closed to all public use between May 15 to June 15 to prevent disturbance of the deer population.
During late summer and autumns of most years, nearly the entire flow of the San Joaquin River is diverted into the Mammoth Pool penstock, essentially drying up nearly 9 miles (14 km) of riverbed. This has resulted in severe declines in fish populations in the stretch between Mammoth Pool and Dam No. 6, where the outflow from the Mammoth Pool powerhouse is located.
Surface area : 1,100 acres (450 ha)
Normal elevation: 3,337 ft (1,017 m) (max)