Mammoth Pool Dam - History

History

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.