The Taum Sauk pumped storage plant is located in the St. Francois mountain region of the Missouri Ozarks approximately 90 miles (140 km) south of St. Louis near Lesterville, Missouri in Reynolds County. The pumped-storage hydroelectric plant, operated by the AmerenUE electric company, was designed to help meet peak power demands during the day. Electrical generators are turned by water flowing from a reservoir on top of Proffit Mountain into a lower reservoir on the East Fork of the Black River. The generators and turbines at river level are reversible, and at night the excess electricity available on the power grid is used to pump water back to the mountaintop. The original upper reservoir had a capacity of 4,350 acre feet (5,370,000 m3). The upper reservoir is 800 feet (240 m) above the hydroelectric plant, which gives it a greater head than that of Hoover Dam. The two are connected by a 7,000 feet (2,100 m) tunnel through the mountain.