Lake Wilhelm (Pa-475) in Mercer, Pennsylvania, is a state-regulated reservoir designed by the USDA NRCS for flood risk reduction along Sandy Creek. Completed in 1969, this earth dam stands at 50 feet in height and spans 500 feet in length, with a storage capacity of 66,400 acre-feet and a surface area of 1680 acres. The primary purpose of Lake Wilhelm is flood risk reduction, with secondary benefits including recreation opportunities for the surrounding community.

Managed by the Pennsylvania Department of Environmental Protection, Lake Wilhelm has a high hazard potential but is currently in satisfactory condition, as assessed during the last inspection in September 2020. The reservoir boasts a drainage area of 56.5 square miles and is a vital resource for water supply and flood control in the region. With its strategic location and effective design, Lake Wilhelm serves as a key component of the local water resource infrastructure, ensuring the safety and well-being of nearby residents during extreme weather events.