The southern third of Louisiana lies no more than 7.6 meters above sea level, and the extensive salt marshes on the central and western coast average less than 1 meter above sea level. Much of the city of New Orleans is below sea level. This land is highly susceptible to the consequences of rising sea levels and increased storm intensity associated with global warming.

New Orleans was founded in 1718 on a crescent of land on the east bank of the Mississippi River that was considered high enough to be safe from tidal surges and hurricanes. This high ground, about 5 meters above sea level, was a natural levee produced by sediment deposited during annual floods.

Although New Orleans remains the highest land along the entire coast of Louisiana, the extensive salt marshes and river delta that once protected the city have eroded during the past century as a result of "improvements" to the river and development associated with the oil and gas industry (Kelley, 1983).

Much of the coast of Louisiana was formed by alluvial deposits of the Mississippi River, and even today the Mississippi and its primary distributary, the Atchafalaya River, have a tremendous impact on the Louisiana coast, carrying 1-2 million metric tons of sediment per day to the Gulf of Mexico. During the past five thousand years, six different outlets formed deltas whose remnants can be identified.

The earliest followed roughly the course of the present-day Atchafalaya River and emptied into what is now Cote Blanche Bay. Much of the sediment was carried by the westward gulf current toward the Texas border. The next outlet was further east, near present-day Terrebonne Bay. The third moved to its westernmost outlet along the general course of today's Bayou Teche. Once again, sediments moved westward, stranding the earlier beaches, called cheniers, behind extensive mudflats.

The fourth delta, far to the east, formed present-day St. Bernard Parish, most of which was underwater during the flooding caused by Hurricane Katrina. The river again moved west along the present course of Bayou Lafourche, and finally, about six hundred years ago, the river moved to its current course and began to form the bird's-foot delta at its mouth. As each delta was abandoned, sediments began to compact, resulting in local land subsidence. The interaction of sediment deposition and wave action formed a series of barrier islands, from the Chandeleur Islands on the east to Marsh Island on the west.

Since the time of French settlement, landowners along the Mississippi were required to build and maintain levees, but it was not until major floods in 1849 and 1850 that national concern was raised for controlling the Mississippi River. In 1882, the U.S. Army Corps of Engineers began levee construction. Following the 1927 flood, the federal government committed to a comprehensive flood-control and navigation program that included levees, floodways, channel improvements, and stabilization.

The entire lengths of the Mississippi River and Atchafalaya Basin in Louisiana are confined by levees. Two breaches are designed into the levee system to provide controlled floodways for diverting high flow. The Old River Control Structure, near the natural confluence of the Red River, diverts up to half the flow of the Mississippi during high flood into the Atchafalaya basin. There, it is divided between the Morganza and West Atchafalaya floodways straddling the Atchafalaya River channel.

About 24 kilometers above New Orleans, a second floodway, the Bonnet Carre' Spillway, can divert more than 7,000 cubic meters per second from the Mississippi River into Lake Pontchartrain to relieve pressure on the New Orleans levees.

The Atchafalaya River formed from the lower Red River in the 1500's, when a new bend in the Mississippi River captured the Red River and most of its flow (Dunne, 2005). Over the years, the Atchafalaya gradually broadened and deepened until it began to capture Mississippi River water even during normal flow. By 1953, 30 percent of the flow moved down the Atchafalaya, and there was concern that the Mississippi would shift course and strand Baton Rouge and New Orleans. The Old River Control Structures, completed in 1963, regulate flow from the Mississippi. In 1973, part of the structure nearly failed during a major flood. It is likely the Mississippi will change course if and when such a failure occurs.

The broad alluvial plain of the Louisiana coast supports a sequence of four marsh types categorized primarily based on elevation above sea level. They range from 24-32 kilometers wide on the west side of the state to more than 80 kilometers wide south of New Orleans. The first 1 to 24 kilometers from the shore, a total of 3,640 square kilometers, is saline marsh dominated by salt-tolerant species. The salt marsh merges gradually into brackish marsh covering 4,850 square kilometers. Another 2,830 square kilometers are intermediate marshes, which grade into nearly 4,850 square kilometers of freshwater marsh. Because of the levees, annual floods no longer cover the marshes with fresh sediments, and erosion and subsidence exceeds land building along the entire coast, except for the mouth of the Atchafalaya River, which has a growing delta.

Although most of Louisiana's oil and gas production is now offshore in the Gulf of Mexico, the first productive well was about 40 kilometers north of the coast, near the town of Jennings. The easiest way to access well sites was to dredge canals and float equipment to the site. Virtually all of the state's marshland is laced with service canals running from the coast, rivers, bayous, or the commercial Intracoastal Waterway. South of the Intracoastal Waterway, there are more than 7,240 kilometers of canals and 11,590 kilometers of bayous. These waterways permit saltwater intrusion into the heart of freshwater marshes, killing intolerant plant species. They also provide water courses through which storm surges can move far inland, eroding the fragile marshes.

As a result of human "improvements" in south Louisiana, approximately 155 square kilometers of coastline is lost every year, and what remains is even more susceptible to storm surges that are expected to increase as a result of global warming. New floodways have been opened through the Mississippi River levees south of New Orleans in an effort to flood the marshes with new sediments so they will grow again. However, the region remains increasingly vulnerable to warming-related weather patterns, and if the sea level rises, it will only become more vulnerable still.

References

1. Dunne, Mike. America's Wetland: Louisiana's Vanishing Coast. Photographs by Bevil Knapp. Baton Rouge: Louisiana State University Press, 2005.

2. Kelley, Joseph T., Orrin H. Pilkey, and Alica Kelley. Living with the Louisiana Shore. Durham, N.C.: Duke University Press, 1983.

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