Fort Randall Dam is a 2 miles long earthen dam which spans the Missouri River and impounds Lake Francis Case, the 11th largest reservoir in the U.S. http://www.nwo.usace.army.mil/Media/Fact-Sheets/Fact-Sheet-Article-View/Article/487642/fort-randall-project-statistics/ The dam joins Gregory and Charles Mix County, South Dakota a distance of 880 river miles (1,416 km) upstream of St. Louis, Missouri, where the river joins the Mississippi River.