Lake William, located in Napa, California, is a privately owned water supply reservoir with a primary purpose of irrigation and water supply. Built in 1960, this earth dam stands at a height of 66 feet and has a storage capacity of 340 acre-feet. With a surface area of 16 acres and a drainage area of 0.51 square miles, Lake William plays a vital role in supporting agricultural activities in the region.

Managed by the Natural Resources Conservation Service, Lake William has a high hazard potential but is currently in satisfactory condition as per the last assessment in September 2017. The dam is regulated and inspected by the California Department of Water Resources, ensuring its safety and compliance with state regulations. The reservoir is situated along Tr Milliken Creek and serves as a crucial water resource for the surrounding area.

Despite its small size, Lake William plays a significant role in water supply and irrigation in Napa, California. With its high hazard potential, regular inspections, and state regulation, the reservoir serves as a crucial asset for the community, highlighting the importance of sustainable water resource management in the face of changing climate conditions.