The Atchafalaya River is a 137 miles (220 km) distributary

Distributary

A distributary, or a distributary channel, is a stream that branches off and flows away from a main stream channel. They are a common feature of river deltas. The phenomenon is known as river bifurcation. The opposite of a distributary is a tributary...

 of the Mississippi River

Mississippi River

The Mississippi River is the largest river system in North America. Flowing entirely in the United States, this river rises in western Minnesota and meanders slowly southwards for to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains...

 and Red River in south central Louisiana

Louisiana

Louisiana is a state located in the southern region of the United States of America. Its capital is Baton Rouge and largest city is New Orleans. Louisiana is the only state in the U.S. with political subdivisions termed parishes, which are local governments equivalent to counties...

 in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. 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.