Vanishing Wetlands

The Mississippi Delta is built up from sediment carried across the continent and deposited in the Gulf of Mexico by the Mississippi River. The river discharges on average about a half million cubic feet of water and sediment per second, the sixth largest discharge worldwide.

The Mississippi River is restless. In just the last 7,000 years, it has shifted along the coast at least four times, each time emptying into the Gulf at a new spot. The abandoned outlets compact under their own weight and sink without the influx of sediment from the river. Mohrig says left alone, the river’s main discharge to the Gulf would shift westward to one of its tributaries, the Atchafalaya River.

In the past century, humans have brought the Mississippi to heel, damming it, redirecting its course and hemming it in with 2,000 kilometers of levees and dikes. As a result, a greater fraction of the sediment carried by the river is now discharged out in the deep ocean, rather than building up wetlands. Add to that the draining of wetlands for development and agriculture and the dredging of canals for hydrocarbon exploration and commercial and recreational boat traffic, and the human impacts on these wetlands has been devastating.

The problems do not end there. As Earth’s oceans warm, sea level rises. The effect is compounded in the delta by subsidence, a natural compacting and sinking of the land. About 25 square miles of wetlands around the Mississippi Delta disappear each year.

The decline in wetlands could have big impacts on the fishing industry, the transportation industry and the survival of migrating waterfowl. It also reduces natural defenses against storm surges such as those caused by Hurricane Katrina.