Union Auxiliary No. 3 Reservoir Report

Last Updated: December 5, 2025

Union Auxiliary No.


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Summary

3, located in Dorrington, California, is a privately owned dam regulated by the Department of Water Resources for the safety of dams. Completed in 1910, this rockfill dam stands at a height of 10 feet and serves multiple purposes including hydroelectric power generation, recreation, and water supply. With a storage capacity of 3,283 acre-feet and a surface area of 215 acres, this dam plays a crucial role in managing water resources in the region.

The dam is associated with four other structures and is part of the Federal Energy Regulatory Commission's oversight for inspections and regulatory compliance. Despite having a low hazard potential, the dam's risk assessment is moderate, indicating the need for ongoing monitoring and risk management measures. With its location on the North Fork Stanislaus River, Union Auxiliary No. 3 contributes to the water management infrastructure in Alpine County, supporting the local community's water needs and providing recreational opportunities for residents and visitors alike.

As a key component of the hydroelectric system in the area, Union Auxiliary No. 3 plays a vital role in meeting the energy demands of the region while also ensuring water supply and recreational activities are maintained. With a long history dating back over a century, this dam continues to be a critical piece of infrastructure that requires regular inspection, monitoring, and adherence to safety regulations to mitigate any potential risks and ensure the safety of nearby communities and the environment.

Year Completed

1910

Dam Length

47

Dam Height

10

River Or Stream

North Fork Stanislaus River -

Primary Dam Type

Rockfill

Surface Area

215

Hydraulic Height

9

Drainage Area

15.2

Nid Storage

3283

Structural Height

9

Hazard Potential

Low

Nid Height

10
       
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.