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The Atchafalaya basin is a 595,000 acre (240,788 hectares) swamp located along the Atchafalaya river in the U.S. state of Louisiana. It is the largest basin swamp in North America. Atchafalaya, pronounced ah-CHA-fa-LIE-ah, is a Choctaw word translated as "long river." The Atchafalaya basin is a major flood control area for southern Louisiana and the city of New Orleans, and also serves as home to a wide range of wildlife. At the same time, it is managed as a National Heritage Area and has several towns within its borders.
In the mid-20th century the Atchafalaya Basin was developed as a major flood diversion area for the Mississippi river north of New Orleans. The Atchafalaya river leaves the Mississippi near Simmesport, Louisiana, carrying 30 to 50 percent of the total water present at the split. From there it travels 130 miles (about 209 km) south to the Gulf of Mexico. In the 1950s containment levees were built parallel to the river on both sides. The levees, which lie a minimum of 15 miles (about 24 km) apart along the course of the river, are the boundaries of the Atchafalaya basin.
Water levels in the basin vary by as much as 15 feet (about 4.5 m) in a normal year because of the varying volume of water carried by the Atchafalaya river. The center channel of the river, which runs through the heart of the basin, can have waves up to 3 feet (about 1 m) tall if there is enough wind. All the smaller waterways in the Atchafalaya basin are bayous, which means the water flow changes direction depending on rainfall and the current height of the river itself.
Running across the northern part of the Atchafalaya basin is the largest remaining stand of bottomland hardwood forest in the US. Through the center of the basin is a band of cypress swamp. Along the Gulf coast, there are coastal marsh grasslands.
The Morganza floodway is the major flow control structure for the Atchafalaya basin. It is designed to divert additional flow from the Mississippi to the Atchafalaya, causing the Atchafalaya to flood out into the basin. This spares the levees and communities further south on the Mississippi. The Army Corps of Engineers, which manages the spillway, considers opening it whenever flow in the Mississippi reaches record levels, but it has only been used twice. In 1973 it was partly opened, and In May of 2011 record flooding forced the Corps to use the full spillway for the first time.