Mammoth Pool Reservoir Report

Last Updated: December 5, 2025

Mammoth Pool, a privately owned dam located in Big Creek, California, serves as a key hydroelectric facility along the San Joaquin River.


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Summary

Built in 1960 by Bechtel, the earth dam stands at a height of 411 feet and has a storage capacity of 122,175 acre-feet. The dam's primary purpose is hydroelectric generation, but it also offers recreational activities on its 1,100-acre surface area.

With a high hazard potential and a moderate risk assessment, Mammoth Pool is regulated and inspected by both state and federal agencies to ensure its safety and compliance with regulations. The dam features an uncontrolled spillway with a width of 403 feet and can discharge up to 235,000 cubic feet per second. Despite its age, Mammoth Pool continues to be a vital water resource and energy provider in the region, contributing to the sustainable development of California's water and energy infrastructure.

Year Completed

1960

Dam Length

820

Dam Height

411

River Or Stream

San Joaquin River

Primary Dam Type

Earth

Surface Area

1100

Hydraulic Height

330

Drainage Area

995

Nid Storage

122175

Structural Height

411

Hazard Potential

High

Foundations

Rock

Nid Height

411
       
Seasonal Comparison

Weather Forecast

Dam Data Reference

Condition Assessment

Satisfactory
No existing or potential dam safety deficiencies are recognized. Acceptable performance is expected under all loading conditions (static, hydrologic, seismic) in accordance with the minimum applicable state or federal regulatory criteria or tolerable risk guidelines.
Fair
No existing dam safety deficiencies are recognized for normal operating conditions. Rare or extreme hydrologic and/or seismic events may result in a dam safety deficiency. Risk may be in the range to take further action. Note: Rare or extreme event is defined by the regulatory agency based on their minimum
Poor A dam safety deficiency is recognized for normal operating conditions which may realistically occur. Remedial action is necessary. POOR may also be used when uncertainties exist as to critical analysis parameters which identify a potential dam safety deficiency. Investigations and studies are necessary.
Unsatisfactory
A dam safety deficiency is recognized that requires immediate or emergency remedial action for problem resolution.
Not Rated
The dam has not been inspected, is not under state or federal jurisdiction, or has been inspected but, for whatever reason, has not been rated.
Not Available
Dams for which the condition assessment is restricted to approved government users.

Hazard Potential Classification

High
Dams assigned the high hazard potential classification are those where failure or mis-operation will probably cause loss of human life.
Significant
Dams assigned the significant hazard potential classification are those dams where failure or mis-operation results in no probable loss of human life but can cause economic loss, environment damage, disruption of lifeline facilities, or impact other concerns. Significant hazard potential classification dams are often located in predominantly rural or agricultural areas but could be in areas with population and significant infrastructure.
Low
Dams assigned the low hazard potential classification are those where failure or mis-operation results in no probable loss of human life and low economic and/or environmental losses. Losses are principally limited to the owner's property.
Undetermined
Dams for which a downstream hazard potential has not been designated or is not provided.
Not Available
Dams for which the downstream hazard potential is restricted to approved government users.