Union Auxiliary No. 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.