Mammoth Pool Dam

Mammoth Power Company

Company type	Private
Industry	Hydroelectric power
Founded	1900
Founder	John S. Eastwood
Headquarters	United States

Mammoth Pool Dam is a hydroelectric dam situated on the San Joaquin River within California's southern Sierra Nevada, roughly northeast of Fresno. It forms the Mammoth Pool Reservoir inside the Sierra National Forest. The dam and reservoir derive their names from a sizable natural pool that once existed above the current dam location.

History

The Mammoth Power Company was established in 1900 by engineer John S. Eastwood and a group of investors aiming to utilize hydroelectric energy from the upper stretches of the San Joaquin River. The Mammoth Pool area, where the river narrows between granite cliffs, was recognized as an optimal site for a dam. Initially, plans called for a rockfill diversion dam to direct water through a tunnel, generating a hydraulic head exceeding to power a 120,000 hp generating station. However, due to the project's ambitious scale, investor enthusiasm waned, leading to the abandonment of the plan by 1901.

Hydroelectric efforts were revived in the mid-1950s, resulting in finalized designs for Mammoth Pool and its associated power facility. Construction commenced in October 1958 after diverting the San Joaquin River via a tunnel bypassing the dam site. Although difficulties arose from granite exfoliation caused by drilling and blasting operations, the dam's construction was completed by late 1959.

Purpose

Primarily designed for hydroelectric power production, the dam represents a crucial component of Southern California Edison's Big Creek Hydroelectric Project, which encompasses 25 dams, nine power stations, and related infrastructure along the San Joaquin River. Water stored in the reservoir is channeled through a reinforced concrete penstock leading to the Mammoth Pool Powerhouse. This facility contains two turbines rated at 100,000 hp each, delivering an installed capacity of approximately 190 megawatts (MW). Due to frictional losses within the tunnel, the effective maximum output is closer to 187 MW. Positioned above the powerhouse is a high surge chamber, which safeguards turbines against abrupt fluctuations in flow. The plant operates with a hydraulic head nearly three times greater than the dam's structural height.

Dimensions

The Mammoth Pool Dam is constructed as a rockfill embankment standing tall from its foundation and extending in length. It towers above the riverbed of the San Joaquin and comprises roughly of material. The crest lies at an elevation of , with an approximate width of , while its base exceeds in thickness.

The reservoir offers a conservation storage volume of , with a total maximum capacity reaching . Its surface elevation can rise to about above sea level. Water discharge is controlled by a cone (Howell-Bunger) valve capable of releasing up to when the reservoir is at full pool. Any surplus water passes over an uncontrolled ogee crest spillway located roughly one-quarter mile west of the dam; this spillway spans and can manage flows near .

Environmental impacts

The construction of the dam flooded an important migration corridor for the Sierra mule deer, compelling them to swim across the reservoir during their seasonal movements. To safeguard this wildlife, public access to the reservoir is restricted annually from May 15 through June 15.

During certain times of the year, almost the entire San Joaquin River flow is redirected into the Mammoth Pool penstock, leaving approximately of the downstream riverbed dry. This water diversion has led to a notable decline in fish populations between Mammoth Pool and Dam No. 6, where the powerhouse discharges its outflow.

See also

• Big Creek (San Joaquin River)
• List of power stations in California
• List of reservoirs and dams in California
• List of the tallest dams in the United States