South Dakota’s Grand River is a tributary of the Missouri. Its combined branch has a length 110 miles, while its longest fork has a length of rough 200 miles. It is formed by the confluence of the North Fork and the longer South Fork in the northwestern part of the state near Shadehill in Perkins County, near sections of the Grand River National Grassland. From there it flows east, through the Standing Rock Indian Reservation and joins the Missouri River in Lake Oahe, around10 miles northwest of Mobridge. The lower 15 miles of the Grand River are an arm of the Lake Oahe reservoir.

Any frontier history buff worth his salt knows that the forks of the Grand River was the site of the infamous 1823 attack by a grizzly bear on legendary frontiersman Hugh Glass. Glass was left for dead by he companions, yet survived to make his way back to civilization (crawling most of the way).