The Atchafalaya Basin,[p] or Atchafalaya Swamp, is the largest swamp in the United States. Located in south central Louisiana, it is a combination of wetlands and river delta area where the Atchafalaya River and the Gulf of Mexico converge. The river stretches from near Simmesport in the north through parts of eight parishes to the Morgan City area in the south. The Atchafalaya is unique among Louisiana basins because it has a growing delta system (see illustration) with nearly stable wetlands.[1]
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The Atchafalaya Basin, the surrounding plain of the river, is filled with bayous, bald cypress swamps, and marshes that give way to more brackish estuarine conditions and end in the Spartina grass marshes, near and at where it meets the Gulf of Mexico. It includes the Lower Atchafalaya River, Wax Lake Outlet, Atchafalaya Bay, and the Atchafalaya River and Bayous Chene, Boeuf, and Black navigation channel. See maps and photo views of the Atchafalaya Deltas centered on .
The basin, which is susceptible to heavy flooding, is sparsely inhabited. The basin is about 20 miles (32 km) in width from east to west and 150 miles (240 km) in length.[2] With 595,000 acres (2,410 km2), it is the nation’s largest swamp wilderness, containing nationally significant expanses of bottomland hardwoods, swamplands, bayous, and back-water lakes.[3] The Basin's thousands of acres of forest and farmland are home to the Louisiana black bear (Ursus americanus luteolus), which has been on the United States Fish and Wildlife Service threatened list since 1992.[4]
The few roads that cross it follow the tops of levees. Interstate 10 crosses the basin on elevated pillars on a continuous 18.2 mile (29,290 m) bridge from Grosse Tete, Louisiana, to Henderson, Louisiana, near at the Whiskey River Pilot Channel at .
The Atchafalaya National Wildlife Refuge was established in 1984 to improve plant communities for endangered and declining species of wildlife, waterfowl, and migratory birds.[5]
Geologically, the Atchafalaya River has served periodically as the main channel of the Mississippi River through the process of delta switching, which has built the extensive delta plain of the river. The natural levees created by earlier main channels border and help define the Atchafalaya Basin, with the Atchafalaya River's natural levee running southward along the western edge of the basin. The central basin is further bordered by man-made levees designed to contain and funnel floodwaters released from the Mississippi at Morganza south toward Morgan City and eventually to the Gulf of Mexico.
Since the early 20th century, because of manmade alterations in the channel, the Mississippi has sought to change its main channel to the Atchafalaya River. By law, a regulated proportion of the water (30%) from the Mississippi is diverted into the Atchafalaya at the Old River Control Structure. In times of extreme flooding, the US Army Corps of Engineers may open the Morganza Spillway to relieve pressure on levees and control structures along the Mississippi. On May 13, 2011, in the face of a rising Mississippi River that threatened to flood New Orleans and other heavily populated parts of Louisiana, the USACE ordered the Morganza Spillway opened for the first time since 1973. This water floods the Atchafalaya Basin between the levees along the western and eastern limits of the Morganza and Atchafalaya basin floodways.
The control of the river's floods, along with those of the Mississippi River, has become a controversial issue in recent decades. It is now widely suspected that the channeling of the river and subsequent lowering of siltation rates has resulted in severe degradation of the surrounding saltmarsh wetlands as well as widespread submerging of populated and agricultural lands of the bayou country. The US Geological Survey (USGS) reports that over 29 square miles (75 km²) of land is lost to the sea each year.[6]
The decrease in water movement and siltation rates has been caused by damming the bayous and laying sluices, by the US Army Corps of Engineers. Also, the placing of a levee along the Atchafalaya Basin has contributed to cutting off the swamp from the river, preventing the swamp from functioning as a buffer. This decrease has caused reduced aeration of the water, causing it to change color. Where the water was formerly brown, it is now black.[7]
The coastal salt marshes form a buffer zone protecting the entire coast of Louisiana from the effects of hurricanes in the Gulf of Mexico by dissipating their accompanying storm surges. The marshes depend on replenishment from deposited silt, which is now being deposited instead over the edge of the continental shelf, due to the artificially canalized flow of the Mississippi. Also, from the 1950s through 1970s, the oil industry dredged deep channels into the marsh so that they could move barges in as work platforms. The edges continued to degrade, until wide shallow channels in the saltmarsh have resulted.[8]
The disappearance of the delta country is considered by many environmentalists, as well as by the State of Louisiana, to be one of the most significant ecological threats in the United States. The loss of the delta lands was discussed by author Mike Tidwell in his 2003 book Bayou Farewell: The Rich Life and Tragic Death of Louisiana's Cajun Coast.[9]
^[p] The word "Atchafalaya" is pronounced as "ah-CHAF-fah-Lie-ah".[10]