The Atchafalaya River is a distributary of the Mississippi and Red rivers, approximately 170 miles (270 km) long, in south central Louisiana in the United States. It is navigable and provides a significant industrial shipping channel for the state of Louisiana, as well as the cultural heart of the Cajun Country. The maintenance of the river as a navigable channel of the Mississippi has been a significant project of the U.S. Army Corps of Engineers for over a century.
It is formed near Simmesport at the confluence of the Red River with the Mississippi, where the Mississippi connects to the Red by the 7-mile (11-km) canalized Old River. It receives the water of the Red as well as part of the water of the Mississippi, which itself continues in its main channel to the southeast. It meanders south as a channel of the Mississippi, through extensive levees and floodways, past Morgan City, and empties into the Gulf of Mexico in Atchafalaya Bay approximately 15 miles (25 km) south of Morgan City. The river is now forming a new delta in the bay – the only place on the Louisiana coastline that is gaining ground.