Union Auxiliary No. 7 Reservoir Report

Last Updated: December 5, 2025

Union Auxiliary No.


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Summary

7, located in Alpine County, California, is a private hydroelectric dam constructed in 1910 along the North Fork Stanislaus River. With a rockfill design and a dam height of 7 feet, the dam serves multiple purposes including hydroelectric power generation, recreation, and water supply. Its primary owner is the Federal Energy Regulatory Commission (FERC), with state regulation by the Department of Water Resources (DWR) ensuring its safety and compliance with permits and inspections.

Covering a drainage area of 15.2 square miles, Union Auxiliary No. 7 has a storage capacity of 3,283 acre-feet and a surface area of 215 acres. The dam's spillway, with a width of 52 feet, is uncontrolled. While the dam has a low hazard potential, a moderate risk assessment (3 out of 5) indicates the need for ongoing risk management measures. Despite its age, the dam's condition assessment is not available, highlighting the importance of regular inspections and emergency preparedness.

Overall, Union Auxiliary No. 7 is a significant water resource infrastructure in California, providing essential services for the region while requiring careful monitoring and maintenance to ensure its continued safety and functionality. Water resource and climate enthusiasts will find interest in its historical significance, engineering design, and ongoing regulatory oversight to protect the surrounding environment and communities.

Year Completed

1910

Dam Length

52

Dam Height

7

River Or Stream

North Fork Stanislaus River -

Primary Dam Type

Rockfill

Surface Area

215

Hydraulic Height

6

Drainage Area

15.2

Nid Storage

3283

Structural Height

6

Hazard Potential

Low

Nid Height

7
       
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.