river formed by the confluence of the North Fork Moreau and South Fork Moreau rivers in Perkins county, northwestern South Dakota, U.S. It flows about 250 miles (400 km) east through the Cheyenne River Sioux reservation, where it receives several north-bank tributaries (including Thunder Butte Creek and the Little Moreau River), to the Missouri River south of Mobridge. The lower Moreau basin was inundated by Oahe Dam, a major flood-control unit of the Missouri River basin project, and is now part of Lake Oahe. The river was named for an early French trader.