Atchafalaya is a Native American word meaning “long river.” It is approximately 170 miles long and is one of the largest distributaries of the Mississippi River. It begins at Simmesport at the confluence of the Red and Mississippi Rivers, and flows through Morgan City to the Gulf of Mexico. Because it is the shortest distance to the Gulf of Mexico, the US Corps of Engineers mandates that only 30% of the flow of the Mississippi River is allowed to flow down the Atchafalaya River.
The Atchafalaya River is an important shipping channel for the state of Louisiana, shortening the distance to the Gulf of Mexico for vessels via Old River and the Gulf Intracoastal Waterway, saving time and money for companies. If the Mississippi River were allowed to flow freely, the Atchafalaya River would capture the main flow of the Mississippi, permitting it to bypass the major port cities of Baton Rouge and New Orleans.The Atchafalaya River is one of the top five rivers in discharge in the United States with an average annual flow of 180,000 cubic feet per second. As a result of draining the most of the US, siltation flowing down the river is building delta at the end of the Atchafalaya River as well as Wax Lake Outlet, an artificial channel off the Atchafalaya River.
Here is a comprehensive map of the expansive Atchafalaya River area: http://atchafalaya.org/maps/AtchafalayaFull.pdf