Lower Peak Lake Auxiliary is a private hydroelectric dam located in Placer County, California, near the town of Washington. Built in 1860, this earth dam stands at a height of 7 feet and has a hydraulic height of 3.5 feet. With a storage capacity of 484 acre-feet, the dam serves the primary purpose of hydroelectric power generation and is regulated by the Federal Energy Regulatory Commission.

Situated on the South Yuba River, Lower Peak Lake Auxiliary has a surface area of 33.3 acres and a maximum discharge of 312 cubic feet per second. The dam features an uncontrolled spillway with a width of 55 feet and has a low hazard potential. Although the condition assessment is not available, the risk assessment is moderate, with a risk level of 3. Emergency action plans have been prepared and last revised in December 2020, ensuring that the dam is well-prepared for any potential hazards.

Overall, Lower Peak Lake Auxiliary is a historic structure with a vital role in providing hydroelectric power to the region. Its location in the picturesque surroundings of Placer County makes it a significant landmark for water resource and climate enthusiasts interested in the sustainable development of water infrastructure in California.