Mississippi diversions fail to rescue shrinking wetlands

Instead, the projects – which involve diverting fresh water from the Mississippi River in the hope of carrying sediment to marshes and aiding plant life – have made these regions more vulnerable to hurricanes, according to the authors of a study published by Geophysical Research Letters.

Soil compaction, geological faulting, and oil and gas drilling are causing the ground to sink. Meanwhile, dams and levees built to constrain the Mississippi have prevented it from depositing the sediments needed to rebuild wetlands.

The Coastal Wetlands Planning, Protection and Restoration Act plans to restore almost 17,000 hectares of marshland over the next two to three decades, at a cost of $1.05 billion. About 65 pc of the projected costs are for freshwater-diversion projects similar to those examined by the study, says lead author Michael Kearney, a coastal scientist at the University of Maryland in College Park.

Kearney and his colleagues analysed Landsat images of three of the longest-running freshwater diversions – the Caernarvon, Naomi and West Point a la Hache diversions – collected between 1984 and 2009. These projects were developed in the early 1990s in an attempt to redirect water and sediments into bays and marshes to help to restore coastal wetlands. But the researchers found that total vegetation and marsh area in the three had not grown significantly.

Moreover, the regions suffered greater damage during Hurricane Katrina than surrounding areas. Most of the new plant growth that has occurred since the diversion was built consists of algae and other floating plants rather than the deep-rooted marsh plants that hold soil in place. This, says Kearney, is due to the influx of nutrients from agricultural run-off and industrial processes.

The researchers conclude that the scientific basis for freshwater diversions is not sufficiently established, and that the emphasis on diversions as a coastal-restoration strategy should be reconsidered. But Douglas Jerolmack, a geophysicist at the University of Pennsylvania in Philadelphia, points to a handful of studies that he says provide extensive modelling, experimental and field data on the value of diversions – data which he says these researchers have disregarded. For example, a 2011 study of coastal-restoration strategies involving diversions points to several success stories.

One of these, originally created for flood control rather than marsh restoration, is the Wax Lake Delta in Louisiana, where more than 100 sq km of wetlands have been built.