Union Auxiliary No. 4 Reservoir Report

Last Updated: December 5, 2025

Union Auxiliary No.


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Summary

4, located in Dorrington, California, is a privately owned rockfill dam completed in 1910 for hydroelectric purposes. This structure stands at a height of 10 feet with a hydraulic height of 9 feet and a length of 70 feet, providing vital water storage of 3,283 acre-feet for the surrounding area. The dam serves multiple purposes including hydroelectric power generation, recreation, and water supply, showcasing its importance in the region.

Managed by the Federal Energy Regulatory Commission, Union Auxiliary No. 4 has a low hazard potential and moderate risk assessment rating. While the condition assessment is not available, the dam is inspected annually to ensure its safety and functionality. The dam's spillway type is uncontrolled, with a width of 70 feet, and its associated structures include CA00426S001, CA00426S002, and CA00426S006. Additionally, the dam is situated along the North Fork Stanislaus River and falls under the jurisdiction of the California Department of Water Resources for regulatory oversight and permitting.

Overall, Union Auxiliary No. 4 plays a crucial role in water resource management and climate resilience in Alpine County, California. With its historical significance and essential functions in providing water storage and hydroelectric power, this dam serves as a key infrastructure supporting the local community's needs and contributing to the sustainable development of the region.

Year Completed

1910

Dam Length

70

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.