Oahe Dam, major unit of the Missouri River basin project, 242 ft (74 m) high and 9,360 ft (2,853 m) long, on the Missouri River, central S.Dak., near Pierre; built 1948-63 by the U.S. Corps of Engineers. The reservoir impounded by the dam extends c.250 mi (400 km) upstream and has one of the largest reservoir capacities in the United States. The dam provides hydroelectric power, flood control, improvement of navigation, and irrigation.