For thousands of years, the Mississippi River dumped rich sediment into the Gulf of Mexico along Louisiana's coast, building the land that is now South Louisiana. Sediment beneath the Louisiana coast runs 60,000 feet thick.

As the sediment compacts naturally, land subsides — 4 to 4.3 feet per century for the deltaic plain and 1.3 to 2 feet per century for the chenier plains in western Louisiana. In addition, sea level is on the rise.

Coastal scientists say land loss is part of deltaic land building. At the same time, the processes that built the land continued to repair and replace what would naturally be lost. Researchers say active land building requires three elements: freshwater flow, new sediment and plant growth. "Until about 1930, the system was in balance," LSU coastal scientist Ivor van Heerden said. But "man upset that balance with navigation and flood control projects."

After the Mississippi River flood of 1927, Congress began a control program still under way today to contain the river with levees and make it more navigable. While it protected those living along its course and improved shipping, it halted the natural land-building processes.

There are no more annual floods replenishing the marshes with fresh water and sediment. The river delivered 463 million tons of sediment a year between 1930 and 1952, but beginning in 1963, the sediment load dropped to 141 million tons. The decrease most likely occurred because of erosion control along the river's banks and dam and reservoir construction on its tributaries.

The lack of sediment and freshwater flow deprived the coastal marsh of its building blocks, the cornerstone of which is plant growth that holds it all together and acts as the base for the next generation’s layers. Ecologist Donald R. Cahoon of the National Wetlands Research Center in Lafayette said when deprived of fresh water, the plants die. "Wetland loss is biological. When root material dies, it all collapses.”

As the 20th Century economy began to expand, navigation channels were cut into the marsh for everything from ocean-going container ships to oil exploration canals used to move equipment in to tap reserves beneath the marsh. Those canals allowed saltwater to move in and kill the freshwater vegetation necessary for healthy marshes.

Studies show about one third of land loss is caused by shoreline erosion, one third by oil and gas impacts, and one third by the effects of levees, impoundments, nutria and other factors. Land eroded at a rate of about seven square miles annually in 1913, increased to 13 square miles annually in the 1930s and peaked at 39 miles square miles per year by the 1970s before slowing to about 25 square miles annually today. However, coastal wetland loss exceeded 44 square miles per year between 1956 and 1978.

The Value of Louisiana’s Coastal Wetlands

Oil and gas infrastructure

More than 80% of the nation's offshore oil and gas is produced off Louisiana's coast, and 25% of the nation's foreign and domestic oil comes ashore on Louisiana roads and waterways. The coastal zone also contains the Louisiana Offshore Oil Port; over 43,000 oil and gas wells, two storage sites for the Strategic Petroleum Reserve; and the Henry Hub, one of the nation’s major natural gas distribution centers. About 30,000 wells are at risk because of land loss.

Shipping

Nearly 3,000 miles of deep and shallow-draft channels are located in the wetlands (Waldemar Nelson, 2002). Five of the nation’s 15 largest ports are located in south Louisiana, and these facilities carry 21% of all waterborne commerce by tonnage in the United States each year (USACE, 2000). An estimated 155 miles of interior waterways are threatened by coastal erosion. In many places, the barge traffic on the waterways is protected by a thin strip of land, the Corps said.

Fisheries

Each year, Louisiana's commercial and recreational fishing industries contribute $3.5 billion and more than 40,000 jobs to the state's economy (Southwick Associates, 1997). Almost one-third of the fish harvested by weight in the lower 48 states comes from Louisiana’s coastal zone (USDOC, 2001). The annual economic impact of recreational fishing is approximately $944 million (Southwick Associates, 1997). A marsh pond produces 40.7 pounds of shrimp per acre and 70.4 pounds of crabs per acre, a fisheries service study in 1992 showed.

Hurricane protection

Marshes act as a barrier for hurricane storm surges, the damaging bulge of water pushed onto land by the storm. For every 2.7 miles of marsh, storm surge is reduced by one foot. Using one estimate, the coast’s 2.5 million acres of wetlands have annual storm protection values of between $520 million and $2.2 billion (Costanza, Farber, and Maxwell, 1989). More than 2 million people live in the coastal zone that could be affected by a hurricane.

Waterfowl

Waterfowl use Louisiana coastal marshes as stopping points during their north and south migrations each year. The marsh is also home to animals that are harvested. In 1997, the value of pelts and wild alligator skins totaled $1.3 million and $93 million, respectively, according to "Coast 2050." In addition, hiking, bird watching, photography, and camping in the coast contribute more than $220 million annually to Louisiana’s economy.