Atchafalaya River

The Atchafalaya River /ətʃæfəˈlaɪ.ə/ is a 137-mile-long (220 km) distributary of the Mississippi River and Red River in south central Louisiana in the United States. It flows south, just west of the Mississippi River. The Atchafalaya River 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 River has been a significant project of the U.S. Army Corps of Engineers for over a century. The river valley forms the Atchafalaya Basin and Atchafalaya Swamp located in southern Louisiana near the Gulf of Mexico.

The river is formed near Simmesport at the confluence of the Red River with the Mississippi, where the Mississippi connects to the Red River by the 7-mile-long (11 km) canalized Old River (part of the Old River Control Structure). It receives the water of the Red as well as roughly 30% of the water of the lower Mississippi (the other 70% continues in its main channel to the southeast). The volume the Atchafalaya receives from the Mississippi is controlled by the Old River Control Structure, a system of dams and spillways near Red River Landing, Louisiana, and in times of extreme flooding the Morganza Spillway further downstream. If the Mississippi were allowed to flow freely, the Atchafalaya would capture the main flow of the Mississippi, permitting the Mississippi to bypass its current path through Baton Rouge and New Orleans. Despite control efforts, some researchers believe the likelihood of this event increases each year due to natural forces inherent to river deltas. The Atchafalaya River meanders south as a channel of the Mississippi, through extensive levees and floodways, past Morgan City, and empties into the Gulf in Atchafalaya Bay approximately 15 miles (24 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.

The name "Atchafalaya" comes from Choctaw for "long river", from hachcha, "river", and falaya, "long".

The river's valley was designated Atchafalaya National Heritage Area in 2006.