In The Field

ESA 2009: Losing Louisiana

waxlakedelta.jpgWhen will society let go of a land that’s lost? Not until long after it should, as I was reminded by a talk by Robert Twilley of Louisiana State University.

Twilley is an expert on the vast delta of the Mississippi River that feeds Louisiana coastal wetlands with both freshwater and rich sediment. Or rather, it used to — until decades of water diversions choked off much of the water supply. Hurricanes such as Katrina and Rita in 2005 have also wiped out hundreds of square kilometres of land. Sea level rise, and land subsidence, conspire further to threaten to drown much of the coast.

A Nature Geoscience paper published in June argues that relative sea level rise will wipe out 10,000 to 13,500 square kilometres of coastal land by the year 2100 — and there’s apparently nothing we can do about it. The Mississippi River has simply been too dammed up and altered for it to ever provide enough sediment back to the delta to rebuild coast or even counter much of the decline. In other words, Twilley told the ESA meeting, we’re past the point of no return. “We’ve decreased the capacity to adapt,” he said. “We are now outside of the adaptation envelope because of the way we’ve mismanaged the river.”

What’s left to do? One idea is to look at how much coast could be restored if we tried as hard as we could. The Wax Lake Delta (pictured) started forming from sediments around about 1973, as a result of water diversions along Louisiana’s Atchafalaya River. Studying how it builds up over time provides one test system to understand the land-building capacity of the coast, Twilley says. Moving a model of how Wax Lake is built to the Mississippi, he says, suggests that 1,000 square kilometres of wetlands could be created in the next century if sea level rise and land subsidence together amount to 7 millimeters per year — a sort of middle-of-the-road estimate.

Society has put its mind to greater tasks, Twilley argues. Whether battling a vanishing coastline is worth it is a difficult question, however. Restoring Louisiana could be a great environmental triumph — or a great societal folly. What do you think?

Image: National Center for Earth-surface Dynamics

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