Cornelia Dean, The New York Times 28 Jun 09;
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.
Levees have turned the river into “a pipe” south of St. Louis, Dr. Roberts said. Getting sediment into the marshes, he said, “is not happening, at least not very efficiently.”
The extent to which inland dams have had an impact on sediment flow has been debated. Although sediment in the river is only about half what it was in the 18th and 19th centuries, some scientists have argued that the flow back then was unusually high because of the advent of farming in the nation’s midsection.
Dr. Roberts said a new analysis of sediment data going back thousands of years challenged that idea. “There probably was a spike, but it was insignificant,” he said.
In theory, it might be possible to remove inland dams to increase the flow of sediment. But Dr. Roberts said the trapped sediment contained agricultural chemicals and other pollutants that might worsen the already deteriorating water quality at the mouth of the Mississippi.
On the other hand, he said, if nutrient-rich sediment made its way into Louisiana’s marshes, it might encourage the growth of plants that would contribute to marsh health.
