Louisiana coast and sea-level rise
The Louisiana coast is particularly vulnerable to the impacts of sea-level rise, with much of its southern region lying only a few meters above sea level. New Orleans, founded on a natural levee created by sediment from the Mississippi River, faces increasing threats as rising waters and intensified storms linked to climate change erode protective marshlands. The extensive coastal marshes, which provide critical habitats and serve as natural buffers against storms, have been significantly diminished due to both natural disasters and human activities, including canal dredging for oil and gas access.
Between 1932 and 2016, Louisiana lost thousands of square kilometers of land, and projections indicate that the state could experience further severe land loss by 2050. The region's levee systems, while designed for flood control, have hindered the natural sediment replenishment necessary for marsh restoration. As climate change continues to accelerate, predictions suggest that certain low-lying areas may experience minor flooding daily by 2100. This alarming trend has prompted initiatives for relocation and resettlement of communities at risk, highlighting the urgent need for comprehensive strategies to mitigate the impact of rising seas and protect the unique ecosystems of the Louisiana coast.
Louisiana coast and sea-level rise
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.
Background
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.
Natural Mississippi River
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 to 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.
Managed Mississippi River
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 US 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, floodwalls, gates, pumps, 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 floods 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 Carré 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 1500s, when a new bend in the Mississippi River captured the Red River and most of its flow. 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.
Marshes
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 to 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 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 exceed land building along the entire coast, except for the mouth of the Atchafalaya River, which has a growing delta.
Oil and Gas
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 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.
Context
Between 1932 and 2016, an estimated 5,197 square kilometers (2,006 square miles) of land were lost from coastal Louisiana, partly due to natural causes such as hurricanes and partly due to human “improvements” in southern Louisiana. This land loss continued to increase throughout the years. By 2024, southern Louisiana was experiencing some of the highest annual rates of land loss in the world. What remains was even more susceptible to storm surges that are expected to increase as a result of global warming. New floodways were 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 remained increasingly vulnerable to warming-related weather patterns, and as the sea level rises, it would only become more vulnerable still. New Orleans built a 1.8-mile-long seawall to help the city withstand 100-year floods, or 1 percent floods, which were becoming more frequent, but will not keep water out during 500-year floods. Many other South Louisiana locales lacked any such protections.
The National Oceanic and Atmospheric Administration (NOAA) predicted in 2018 that the Louisiana coast could see minor floods daily by 2100 because of increasing subsidence and rising seas. Climate disruption was also expected to bring stronger rains and hurricanes to the region. By 2022, NOAA had predicted that the state would lose more than 4,000 square miles of land by 2050. For those reasons, the federal government has awarded grant funds to relocate the Biloxi-Chitimacha-Choctaw Indians and, in 2017, the state drafted a plan for buyouts in high-risk areas and resettlement of coastal residents farther inland.
A study published in the Washington Post in 2024 predicted the collapse of Louisiana's wetlands, a natural buffer against hurricanes. Significant sea-level rise over the past 13 years had left most of the state's wetlands drowning or expected to drown if the sea level continued to rise for ten or twenty more years. In addition to protecting the state from storms, wetlands provide habitats for fish and birds, store carbon, and filter pollutants from the water. The loss of wetlands has brought the ocean closer to New Orleans and other populated areas.
Key Concepts
- barrier island: an offshore island running parallel to a coastline that protects the coastline from storm surges
- bayou: a small, often intermittent, distributary
- delta: a network of distributaries through deposited sediments at the mouth of a river
- distributary: a branch of a river that removes water from the main river, especially near or in a delta
- levee: a natural or human-made raised bank along a river
- saltwater intrusion: inland movement of seawater
- subsidence: land sinkage due to compaction of underlying material
Bibliography
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