Atchafalaya Swamp

• Category: Inland Aquatic Biomes.
• Geographic Location: North America.
• Summary: The largest river swamp in the United States, the Atchafalaya Swamp is one of the last great wildernesses and contains a rich variety of plant and animal species.

The Atchafalaya is the largest river swamp in the United States. It is contained within the extensive Atchafalaya Basin—3,222 square miles (8,345 square kilometers) of the Mississippi River deltaic plain in south-central Louisiana. The basin begins near Simmesport, Louisiana, where the Mississippi, Red, and Atchafalaya Rivers meet, and extends 137 miles (220 kilometers) south to the Gulf of Mexico. The Atchafalaya River is the fifth-largest by discharge in North America and the largest distributary, or outlet stream, of the Mississippi River, which is its greatest water source.

A navigable river, the Atchafalaya provides a significant industrial shipping channel. Since 1928, the Atchafalaya Basin has also been part of the Mississippi River & Tributaries Project (MR&T), a program of the U.S. federal government that provides flood protection for millions of people. Flood-protection levees confine the Atchafalaya's original floodplain to three floodways that are just half of the basin's original size. The Atchafalaya remains one of the last great wilderness areas in the United States, however, with marshes, cypress swamps, and bottomland hardwoods interspersed with bayous and a rich variety of animal species.

Delta-Building Activity

For the past 10,000 years, delta-building in the Mississippi River system has consisted of cyclic construction and abandonment of delta lobes, with each major lobe being active for 1,000 to 1,500 years. The Mississippi River built southern Louisiana, depositing sediment from upstream and changing river course periodically, looking for a shorter route to the Gulf of Mexico. Large basins such as the Atchafalaya Basin were formed between current and old Mississippi River channels and their natural levees. A dynamic system, the Atchafalaya swamp was shaped by this systematic series of delta-building events. The basin contains one of the few remaining largely natural deltaic ecosystems in the world.

With the Atchafalaya River's progressive capture of more flow of the Mississippi River since the 1500s, the Atchafalaya is currently in a delta-building phase. The Atchafalaya River provides a much more efficient route (137 miles or 220 kilometers) for water and sediment to reach the Gulf of Mexico than the current Mississippi River course (323 miles or 520 kilometers). The natural boundaries of the Atchafalaya Basin are the modern Mississippi River levee to the east and the levees of the Teche (an old course of the Mississippi River) to the west.

From the 1500s to the early 1900s, the large Atchafalaya Basin filled mainly with swamp deposits, and from the Atchafalaya River, sediment filling in lakes. The Atchafalaya River's increasing capture of flow prompted the U.S. Army Corps of Engineers to build the Old River Control Structure, another feature of the MR&T. Completed in 1963, this floodgate system was built at the meeting point of the two rivers in an attempt to prevent total capture and to maintain the inflow of the Mississippi to the Atchafalaya at 30 percent. Starting in the 1950s, the Atchafalaya River increasingly deposited sediment at the coast, bypassing the basin, which had infilled. After a large flood in 1973, both the Atchafalaya and Wax Lake outlets developed sand-rich bars that were exposed during low tide and rapidly developed into two new deltas, some of the newest land in the world.

Forest and Wildlife

Bottomland hardwood forest grows at the highest elevations in the northern part of the basin. Extensive swamps made primarily of bald cypress and water tupelo are found in the middle of the basin, which is regularly flooded. Freshwater and brackish marshes occur in the lower region, where the Atchafalaya River meets the Gulf of Mexico, and include the habitats of the newly formed Wax Lake and Atchafalaya.

The geological processes of delta formation are still transforming the region. Representing the largest contiguous bottomland hardwood forest and overflow alluvial swamp remaining in the United States, the Atchafalaya provides habitat for wildlife species such as alligators, bald eagles, and crawfish, as well as some endangered species, such as the Louisiana black bear and pallid sturgeon. Designated an Important Bird Area of global significance by the National Audubon Society, the Atchafalaya is a key component of the Central and South American flyway for migratory birds.

Effects of Human Activity

The bountiful resources of the Atchafalaya provided sustenance for Indigenous Peoples and, in the 18th century, for thousands of Cajuns who settled in the basin. Inhabitants adapted their lives to the changing conditions of water levels, often living in houseboats. The large 1927 flood devastated many communities and prompted people to migrate from the basin. Almost half of the Atchafalaya Basin Floodway is publicly owned. The rest is privately owned land that includes upland forests and deepwater swamps. The basin is now sparsely populated, but loggers and fishermen still use the basin's resources for their livelihoods. The rich biodiversity also makes the Atchafalaya highly valued by hunters and other sportsmen, photographers, and birders.

Extensive logging has depleted the old-growth cypress trees in the basin, and oil and gas activities have created numerous canals that cut through the swamp, bringing in saltwater. It has also created "spoil banks," piles of earth that rise 10 feet or higher and block the natural flow of water. Experts contend that it is necessary to backfill the canals or provide more gaps in the spoil banks to let more water through. However, many private landowners who earn a profit from oil companies do not want the banks altered.

The basin also has compromised water quality from pollutants, again due to the changes in natural water-flow patterns, which, coupled with changes in sediment deposition, have led to the loss of bald cypress. Further, years of hydrological manipulation have negatively impacted the river and surrounding lands. Restoration plans are underway to restore natural hydrology and to improve the ecological health of the Atchafalaya Basin and the Mississippi River Delta Plain. Some of these measures incorporate the secondary threats of climate warming, including sea-level rise and greater storm surges—another salt intrusion factor. Another element is the vast increase in nitrogen deposition throughout the delta from upstream farmland runoff, itself exacerbated by documented heavier summer precipitation in the Upper Mississippi River Basin, which in turn is in part accelerated by higher global temperatures (which cause more water to evaporate from ocean surfaces, thus contributing to greater precipitation). Along with other strategies, the Atchafalaya National Wildlife Refuge Comprehensive Conservation Plan of 2011 outlines mitigation efforts to reduce and clean up pollutants from the oil industry, agricultural sources, and other human activities.

Bibliography

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