Union Main Reservoir Report

Last Updated: December 5, 2025

Union Main, also known as Union No.


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Summary

1, is a privately owned dam located in Alpine County, California, along the North Fork Stanislaus River. Built in 1910, this rockfill dam stands at 36 feet high and spans 1142 feet in length, with a storage capacity of 3283 acre-feet. The primary purpose of Union Main is hydroelectric power generation, with additional benefits for recreation and water supply.

Managed by the Federal Energy Regulatory Commission, Union Main is regulated at both the state and federal levels, with regular inspections to ensure its safety and compliance with regulations. Despite its low hazard potential, the dam is considered to have moderate risk (3) and has an emergency action plan in place. The dam's surroundings offer a surface area of 215 acres for recreational activities, making it a vital resource for the local community.

Located in a picturesque setting near Dorrington, California, Union Main serves as a critical infrastructure for water resource management in the area. With its historical significance and multifaceted benefits, this dam continues to play a crucial role in supplying water, generating clean energy, and providing recreational opportunities for residents and visitors alike.

Year Completed

1910

Dam Length

1142

Dam Height

36

River Or Stream

North Fork Stanislaus River -

Primary Dam Type

Rockfill

Surface Area

215

Hydraulic Height

29

Drainage Area

15.2

Nid Storage

3283

Structural Height

34

Hazard Potential

Low

Nid Height

36
       
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.