Land Sinks, Waters Rise, Coastal Wetlands Disappear

FOR 7,000 YEARS, THE MISSISSIPPI RIVER flooded Louisiana’s coast with land-building sediment.

The amount of new land this sediment created always exceeded the amount of land lost to the natural processes of subsidence, erosion and sea-level rise. Then in the 20^th century, raising the levees erected along the banks of the river prevented its sediment from reaching the coastal marshes. This upset the balance between land lost and land gained, and began the catastrophic retreat of Louisiana’s wetlands.

Human Actions Amplify Natural Forces

If only deprived of replenishing sediment, the coastal wetlands would eventually succumb to the processes that cause land loss. But human activity speeds the wetlands’ disappearance by exacerbating the effects of these natural processes.

Subsidence, the sinking of land

The loose soils of Louisiana’s coast naturally compact and sink, or subside. Draining land for agriculture, road and canal construction and urban development removes the water’s volume from the soil, speeding the rate of compaction. The weight of buildings, levees and spoil banks further compresses the soil. Disrupting the natural hydrology, protective levees and flood control structures prevent waters carrying nutrients and sediment from replenishing the landscape. According to a report released by the U.S. Geological Survey, without accreting sediment to offset lost elevation, subsidence has accounted for 53 percent of land loss in Louisiana’s deltaic plain over the past century.

Subsidence also occurs as geologic faults cause layers of earth to shift and settle deep below ground. Some observers believe energy extraction activates faults when chambers beneath the coastal area, emptied of oil and gas, fall in and cause underground strata to collapse.

Erosion, the wearing away of land

The abrasive force of water and wind naturally wears away, or erodes, land. Wherever land and water interface, erosion may occur.

Human manipulation of the landscape has multiplied the force of erosion in wetlands already weakened by sediment deprivation. In the past century, canals built in Louisiana’s marshes for navigation and oil and gas activities have created mile upon mile of new stretches where land and water meet. Waves generated by wind and boat wake wash away the edges of these new shorelines, stirring up sediment and carrying it off. The canals also serve as conduits through which salt water enters interior marshes and weakens freshwater vegetation. Without plants to hold it in place, marsh soil disintegrates, hastening the conversion of wetlands to open water.

Sea-level rise, the swamping of land

Historically, some natural fluctuation occurs in the average level of the sea relative to lands but, as a result of human-caused global climate change, scientists predict a sudden and dramatic sea-level rise in the coming century. As mean temperatures climb, polar ice melts, ocean currents change and winds shift, the Louisiana coast could experience average seas nearly two feet higher than at present. Mere inches of elevation could determine the loss or survival of threatened wetlands.

Returning to the River

As they search for ways to halt the destruction of Louisiana’s coastal wetlands, scientists look to the river for help and hope. Allowing the Mississippi to revert to flooding wetlands beyond its levees with its nutrient- and sediment-rich waters would help to alleviate the crisis of land loss, but at the cost of the human community, the infrastructure and commerce of coastal Louisiana. Instead, scientists and engineers seek to replenish the wetlands, yet retain vital flood protection, by controlling the delivery of sediment into the marshes. This can be done by diverting river water through man-made channels, or by capturing the sediment and moving it to project sites.

Draining over 40 percent of the continental United States, the Mississippi River collects sand, silt and clay to become a huge, sustainable source of sediment. The challenge for wetland restoration experts is to capture a portion of the tons of particles carried in the river and transport it to create new land and to nourish existing marshes. Pumps, pipes and dredges can combine to restore some of the benefits that the river’s floods historically delivered to coastal Louisiana.