river rising in the St. Francois Mountains, southeastern Missouri, U.S., and flowing south to the edge of the Ozark Plateau at Wappapello, Mo. It continues across the Mississippi alluvial plain to enter the Mississippi River just above Helena, Ark., after a course of 425 mi (684 km). For 40 mi the river forms part of the Missouri–Arkansas boundary. In Wayne County, Mo., the river is impounded by Wappapello Dam (built in 1941). Heavy rainfall in the Ozarks, which make up 70 percent of the river's drainage basin of 8,400 sq mi (21,800 sq km), runs off rapidly and despite the dam still causes floods in the lower basin. In 1960, as part of a U.S. Army Corps of Engineers project, levees and ditches were constructed and channels realigned between Wappapello and the river's mouth.