Beardsley Afterbay Reservoir Report

Last Updated: December 5, 2025

Beardsley Afterbay, located in Sonora, California, serves as a vital hydroelectric resource along the Middle Fork Stanislaus River.


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Summary

Built in 1958, this rockfill dam stands at a height of 33 feet, containing a maximum storage capacity of 355 acre-feet. With a primary purpose of hydroelectric power generation, Beardsley Afterbay plays a crucial role in providing sustainable energy to the region.

Managed by a public utility entity and regulated by the Department of Water Resources and the Federal Energy Regulatory Commission, Beardsley Afterbay boasts a low hazard potential and moderate risk assessment rating. Despite its age, the dam has undergone regular inspections and meets state jurisdiction and permitting requirements. While the spillway type is uncontrolled, the dam's structural integrity remains sound, ensuring the safety of the surrounding communities.

With its picturesque surroundings and functional design, Beardsley Afterbay stands as a testament to the intersection of water resource management and climate sustainability. As a key player in the local hydroelectric infrastructure, this dam continues to harness the power of the Middle Fork Stanislaus River while adhering to stringent regulatory standards and ensuring the safety of its operations.

Year Completed

1958

Dam Length

240

Dam Height

33

River Or Stream

Middle Fork Stanislaus River

Primary Dam Type

Rockfill

Surface Area

26

Hydraulic Height

31.2

Nid Storage

355

Structural Height

33

Hazard Potential

Low

Nid Height

33
       
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.