The Atchafalaya Basin is the largest river swamp in the United States: about 5,700 km² (1.4 million acres) of cypress–tupelo swamp, bottomland hardwood forest, bayous, lakes and marsh in south-central Louisiana, stretching roughly 240 km (150 mi) from north to south and about 32 km (20 mi) across. It follows the Atchafalaya River, a distributary that takes 30 percent of the combined flow of the Mississippi and Red rivers at Simmesport and carries it to the Gulf of Mexico in Atchafalaya Bay, south of Morgan City. About 70 percent of the basin is forest and 30 percent marsh and open water, and its 1,050 km² (260,000 acres) of cypress is the largest contiguous tract of coastal cypress left in the country. It is not a national park; protection is a patchwork that includes the 177 km² Sherburne Complex Wildlife Management Area, which takes in the Atchafalaya National Wildlife Refuge, Lake Fausse Pointe State Park, and the Atchafalaya National Heritage Area designated in 2006. The simplest way to understand it is to follow the water from top to bottom.
Old River and Simmesport: where the Mississippi is held back
The journey starts at the Old River Control Structure, a cluster of floodgates on the west bank of the Mississippi in central Louisiana, near Red River Landing. Its job is to stop the Mississippi from doing what it wants to do. In the 15th century a westward loop of the Mississippi, later called Turnbull's Bend, cut into the Red River near present-day Angola, and the lower Red became a branch of the Mississippi. In 1831 the river engineer Henry Shreve dug a canal across the neck of the bend to save steamboats a 32 km detour, and between 1839 and 1840 locals cleared the Great Raft, a log jam that had blocked the Atchafalaya. From then on the Atchafalaya, a shorter and steeper route to the Gulf, captured more and more of the Mississippi. Its share of the flow passing Red River Landing rose from under 10 percent in 1850 to about 30 percent by 1950, and in 1953 the US Army Corps of Engineers warned that the Mississippi could switch course entirely by 1990.
The Corps finished the control structure in 1963. The 1973 Mississippi flood nearly undermined it, and an auxiliary structure was added in 1986; the complex also holds a navigation lock and the Sidney A. Murray Jr. Hydroelectric Station. By law, the gates pass 30 percent of the combined flow of the Mississippi, Red and Black rivers into the Atchafalaya. In the 2011 flood they were discharging more than 20,000 m³ (706,000 cubic feet) per second. Just downstream, the Atchafalaya River proper begins at Simmesport, a town of 1,468 people in 2020 and the northernmost on the river. From here it runs 220 km (137 mi) to the sea, and by discharge it is the fifth-largest river in North America. Its name comes from the Choctaw for "long river".
The Morganza Floodway: opened in 1973 and 2011
Further downstream, at river mile 280 of the Mississippi near Morganza in Pointe Coupee Parish, is the basin's second gate. After the Great Mississippi Flood of 1927, the whole Atchafalaya Basin was designated an official floodway, and the Morganza Control Structure was completed in 1954. When the Mississippi threatens Baton Rouge and New Orleans, the Corps can open it and send water down the Morganza Floodway into the basin. It has been opened only twice: in 1973 and on 14 May 2011, after the order was given on 13 May. In 2011 the floodway ran at about a quarter of its capacity, spreading Mississippi water through the swamp between Morganza and the Gulf. The central basin is now enclosed between man-made protection levees on its eastern and western sides, built so that the swamp could take floodwater without drowning the towns around it.
Krotz Springs and Bayou des Ourses: the refuge complex
US Highway 190 crosses the river at Krotz Springs, a town of 904 people in 2020. It grew around a sawmill set up by the Ohio-born businessman Charles William Krotz, who in 1900 drilled for oil and struck water instead, then bottled and sold the spring. The first bridge here opened in 1928, carrying both trains and cars, and was replaced in 1934, 1973 and 1988.
East of the river and stretching from just north of old Highway 190 to a short distance south of Interstate 10 lies the Sherburne Complex, 177 km² (43,637 acres) managed by the Louisiana Department of Wildlife and Fisheries. It was created in a partnership begun in 1983 by combining the Sherburne Wildlife Management Area, the 62 km² (15,220-acre) Atchafalaya National Wildlife Refuge and the Corps of Engineers land at Bayou des Ourses, the "Bayou of the Bears". The name fits. The Louisiana black bear, a subspecies found mainly in the Mississippi valley and the Atchafalaya, was listed as threatened under the Endangered Species Act from 1992 until April 2016 and the basin's forests and farmland are part of its home range. Hunting has since resumed under state permits, with 10 bears taken in 2024 and 16 in 2025.
Whiskey Bay to Henderson on the 29 km I-10 bridge
Most travellers see the basin from Interstate 10, which crosses it on the Atchafalaya Basin Bridge between Baton Rouge and Lafayette. The parallel spans run 29.3 km (96,095 ft), making it the third-longest bridge in the United States; it opened in 1973 and carried about 30,420 vehicles a day in 2015. The bridge has just two exits in the swamp, at Whiskey Bay (Louisiana Highway 975), and at Butte La Rose (LA 3177), and the two spans merge where they cross the Whiskey Bay Pilot Channel and the Atchafalaya River itself. The bridge runs from Grosse Tête in the east to Henderson in the west, a St. Martin Parish town of 1,617 people in 2020.
What the bridge crosses is a swamp in the middle of filling up. Once the Mississippi's sediment-rich water was connected to the basin, silt began settling in the backwater lakes. Open water in the basin shrank from about 490 km² in 1850 to 290 km² in 1950 and 190 km² by 2005, and after the flood-control levees were finished in the 1940s the sediment was confined to an area about one-third the size of the original basin. Between 1960 and 1980, oil and gas companies dredged access and pipeline canals 30 to 50 m wide through the deep swamp; in places there are 2 km of canal for every kilometre of natural bayou. Where those canals open into lakes, the US Geological Survey has measured up to 30 cm (10 in) of sediment a year. Meanwhile the share of flow leaving the main channel to wash across the floodplain fell from the 30 percent planned between 1950 and 1974 to about 13 percent, leaving large areas of stagnant, low-oxygen water.
Lake Fausse Pointe and the buried town of Bayou Chene
South of the interstate, the western edge of the basin follows the old natural levee of Bayou Teche, a former course of the Mississippi. Just inside it, about 29 km east of St. Martinville, is Lake Fausse Pointe State Park, 24 km² (6,000 acres) of former Chitimacha land, where black bears, alligators and bobcats live and where the levees froze a small lake delta in place by cutting off the Grand Bayou distributary.
Out in the middle of the swamp lies a reminder of how fast the basin fills. Bayou Chene was a logging, trapping and fishing community from about 1830; the 1860 census counted 675 residents, and by the early 20th century it was the centre of the basin's cypress and fur trade. The 1927 flood rose seven feet above the natural levees and covered it for weeks. After the basin became an official floodway, high water returned again and again, the post office closed in 1952, and families moved out to New Iberia, St. Martinville and Breaux Bridge. The town now lies under about 3.7 m (12 ft) of silt. For more on the waterways people here lived along, see our guide to Louisiana's bayous.
Morgan City and the new deltas in Atchafalaya Bay
The river's last stretch passes Morgan City, a city of about 10,800 people in 2024 where the Atchafalaya meets the Gulf Intracoastal Waterway. Originally named Tiger Island and later Brashear City, it was renamed in 1876 for Charles Morgan, the rail and steamship owner who first dredged the Atchafalaya Bay Ship Channel for ocean-going vessels. To protect the city from floods, in 1942 the Corps dug the Wax Lake Outlet, about 24 km long from Six Mile Lake, to divert 30 percent of the river's flow to the Gulf further west, with a design capacity of about 12,000 m³ per second.
The river finally reaches Atchafalaya Bay about 24 km south of Morgan City, and here the basin does something no other part of the Louisiana coast does: it gains land. Two young deltas are growing into the bay, one at the river mouth and one at the Wax Lake Outlet, which receives about 34 million tons of sediment a year and pushed about 8 km out to sea between 1941 and 2005. While the rest of the coast loses wetland, these lobes are building it, and they are the newest chapter in the much longer story of the Mississippi River Delta. To place the Atchafalaya among the state's other big waterways, from the Red to the Sabine, see our overview of rivers in Louisiana.