Union of Concerned Scientists

The Mississippi delta region—with big population centers such as New Orleans—represents one of the most vulnerable regions of the Gulf Coast. The combined effects of engineered and altered landscapes, natural subsidence, and climate change will have tremendous consequences for human well-being, natural resources, and biodiversity.

Over the past century, the nearly 1.3 million square mile watershed of the Mississippi River has experienced major environmental changes, including conversion of more than 80 percent of forested wetlands to agriculture and urban areas, channelization, dam construction, and river levees. The construction of massive structures that keep the river from switching channels has restricted sediment and freshwater supply to the flood plain.

Losing Wetlands
These changes have been especially damaging to the region's wetlands. The coastal wetlands associated with the Mississippi River delta make up nearly 40 percent of the total coastal salt marsh in the lower 48 states of the U.S. These wetlands are disappearing at an average rate of 25 square miles per year, about 50 acres each day. Already, more than one thousand square miles of freshwater wetlands in Louisiana have been lost or converted to other habitats. Only about 20 percent of the original bottomland hardwood forests and swamps in the lower Mississippi River valley remain today.

Some of these wetland losses are due to delta subsidence (sinking), which results in relative sea-level rise. Although subsidence is a natural process, human interference with river and sediment flow and withdrawal of groundwater have exacerbated it.

Future Changes
One of the most critical climate change issues for the delta is how the already highly altered and rapidly degrading coastal wetlands of Louisiana will be impacted. Landscape models project that the combined effects of increased sea-level rise and human alterations to sediment supply will continue the current wetland loss rates in the Terrebonne and Barataria basins into this century, This loss of wetlands threatens navigation and the industrial, agricultural, and fisheries sectors of the regional economy. Future wetland loss rates could increase as sea-level rise accelerates. Although hurricanes can play a positive role in natural wetland development by remobilizing sediments and assisting in building marsh substrate, channelized coastal landscapes such as Louisiana's allow storm surges to move farther inland, increasing saltwater penetration into freshwater marshes. Sea-level rise will exacerbate this whether or not hurricanes and storms become more frequent or intense.

Major changes in the upper and lower watersheds of the Mississippi-Atchafalaya river system have also increased the likelihood of low-oxygen conditions (hypoxia) in Gulf waters.

• Wetland destruction has removed the watershed's nutrient-buffering capacity;
  
  
• River channelization for flood control and navigation has shunted fresh water to the continental shelf.
  
  
• Major increases in nitrogen fertilizer washed into the Mississippi River Basin have quadrupled nitrate concentrations in the river system.

On the Louisiana continental shelf, hypoxia can extend over thousands of square miles, depending on the amount of freshwater discharge, which controls the addition of nutrients and the stratification of coastal waters. Climate model studies predict a 20 percent increase in discharge from the Mississippi River watershed, which could lead to an expanded hypoxic or "dead zone."