How Mississippi River floods could save Louisiana's sinking coasts

The network of levees and spillways along the lower Mississippi River have, so far, contained the floods according to plan. For the marshlands of southern Louisiana, at least, that is bad news.

Built in the aftermath of the great Mississippi River flood of 1927, the current levee and spillway system has prevented disastrous floods in Louisiana and kept the river on its current course. But it has also ensured that the massive quantities of silt borne by the Mississippi have been carried straight out the chute of the river's mouth and into the deep waters of the Gulf.

Previously, the Mississippi largely wandered wherever it liked, with floods replenishing marshlands up and down the coast with the sediments they carried. Deprived of those sediments for the past 80 years, the marshy Louisiana coast has steadily subsided, sinking into the Gulf of Mexico.

Now, with the Mississippi having been loosed through the Morganza spillway into the Atchafalaya Basin, scientists are using the great flood of 2011 as an opportunity to study how these waters might replenish wetlands when they reach the coast. Moreover, they are promoting the idea of building more such spillways, which would not only help contain future flooding, but also would help strategically replenish wetlands along more of the Louisiana coast.

“In the Delta region, we trade off preventing regular, small-scale floods with having infrequent but catastrophic large floods,” says Douglas Jerolmack, professor of earth sciences at the University of Pennsylvania. “And by preventing the river from flooding most of the time, we’re also preventing the natural deposit of silt in the marshes needed to maintain the wetlands.”

Protecting New Orleans

Without the Old River control station and the Morganza spillway north of Baton Rouge, La., it is possible that sometime in the past 30 years the Mississippi could have changed course completely – captured by the Atchafalaya River, which offers a steeper, more direct route to the Gulf, Dr. Jerolmack says.

As of Wednesday, the Mississippi had crested at 17.1 feet at New Orleans and was expected to remain at that level through the rest of the week. While the metro New Orleans area seems to be safe, however, as many as 25,000 people who live near the Atchafalaya River Basin west of the city have evacuated in anticipation of flooding from the opening of the Morganza spillway.

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