The Moreau River is a tributary of the Missouri
River, approximately 200 mi (320 km) long, in South Dakota in
the United States. It rises in two forks in northwestern South
Dakota, in the Badlands of Harding County.
The North Fork rises approximately 10 mi
(16 km) northeast of Crow Buttes. The South Fork rises
approximately 20 mi (32 km) west of the headwaters of
the North Fork. The two forks flow ESE and unite near
Zeona in southern Perkins County. The combined stream
flows east, past Usta, and across the Cheyenne River
Indian Reservation, past Iron Lightning, Thunder Butte,
Green Grass and Whitehorse. It joins the Missouri in
Lake Oahe, with the lower 25 mi (40 km) of the river
forming an arm of the reservoir.