Union Auxiliary No. 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.