The Atchafalaya River 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, and is the fifth largest river in North America, by discharge. The name Atchafalaya comes from Choctaw for 'long river', from hachcha, 'river', and falaya, 'long'.

The Red River, or sometimes the Red River of the South to differentiate it from the Red River in the north of the continent, is a major river in the Southern United States. It was named for its reddish water color from passing through red-bed country in its watershed. It is known as the Red River of the South to distinguish it from the Red River of the North, which flows between Minnesota and North Dakota into the Canadian province of Manitoba. Although once a tributary of the Mississippi River, the Red River is now a tributary of the Atchafalaya River, a distributary of the Mississippi that flows separately into the Gulf of Mexico. This confluence is connected to the Mississippi River by the Old River Control Structure.