The Moreau River is a tributary of the Missouri River, approximately 200 mi long, in South Dakota in the United States. Moreau River has the name of a pioneer trader. It rises in two forks in northwestern South Dakota, in the Badlands of Butte and Harding Counties. The North Fork rises approximately 10 mi northeast of Crow Buttes. The South Fork rises approximately 20 mi 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 of the river forming an arm of the reservoir. The river generally parallels its neighbor 40 miles to the north, the Grand River, running from west to east to join the Missouri. Draining the Pierre Hills and Northern Plateaus, the Moreau (formerly Owl River) has a drainage basin of approximately 5400 sqmi. The Moreau River also has Sand Creek as a source, along with the North and South Forks, and claims several tributaries, such as the Little Moreau, Deep, Red Earth, Antelope, and Thunder Butte Rivers. At Promise, the river averages a flow of 296 cuft/s.

Variant names

According to the Geographic Names Information System, it has also been known historically as: