Cornelia Dean in the New York Times writes Dams Are Thwarting Louisiana Marsh Restoration, Study Says. She describes recent research by Michael Blum and Harry Roberts Drowning of the Mississippi Delta due to insufficient sediment supply and global sea-level rise (doi:10.1038/ngeo553) that estimates that dams have reduced sediment outflows by 50% reducing the potential for New Orleans wetland restoration:
Desperate to halt the erosion of Louisiana’s coast, officials there are talking about breaking Mississippi River levees south of New Orleans to restore the nourishing flow of muddy water into the state’s marshes.
But in a new analysis, scientists at Louisiana State University say inland dams trap so much sediment that the river no longer carries enough to halt marsh loss, especially now that global warming is speeding a rise in sea levels.
As a result, the loss of thousands of additional square miles of marshland is “inevitable,” the scientists report in Monday’s issue of Nature Geoscience.
The finding does not suggest it would be pointless to divert the muddy water into the marshes, one of the researchers, Harry H. Roberts, said in an interview. “Any meaningful restoration of our coast has to involve river sediment,” said Dr. Roberts, a coastal scientist.
But he said officials would have to choose which parts of the landscape could be saved and which must be abandoned, and to acknowledge that lives and businesses would be disrupted. Instead of breaking levees far south of New Orleans, where relatively few people live, Dr. Roberts said, officials should consider diversions much closer to New Orleans, possibly into the LaFourche, Terrebonne or St. Bernard basins.
“It’s going to be an excruciating process to decide where that occurs,” Dr. Roberts said of the levee-breaking.
Sediment carried by the Mississippi built up the marshes of Louisiana over thousands of years, but today inland dams trap at least half of it, Dr. Roberts said. He pointed out that there were 8,000 dams in the drainage basin of the Mississippi.
In their article Blum and Roberts conclude that significant sea level rise is inevitable even if sediment loads are restored, because sea level is now rising at least three times faster than the building of the Mississippi delta.