Scientists: Water rising too fastto save good portion of delta

The state of Louisiana is proceeding with ambitious plans to redirect the Mississippi River and rebuild some of its rapidly vanishing wetlands — but even this massive intervention might not be enough to save the most threatened lands from fast rising seas, scientists concluded in a study published Wednesday.

The study uses a methodology called "optical dating" to study how the river built an area called the Lafourche subdelta in coastal Louisiana, where the Mississippi dumped loads of sediment up to about 600 years ago, when it changed paths. The technology lets scientists identify the last time that long-buried sand was actually exposed to sunlight, and therefore determine the rate at which the river naturally built up land by carrying sediment downstream.

"What we found was that, on average, it produced somewhere between 6 and 8 square kilometers of land per year, and the shoreline migrated seaward by somewhere between 100 and 150 meters per year," said Torbjorn Tornqvist, a Tulane University geologist who was one of the study's authors. "Those numbers in themselves I find pretty impressive."

"But the problem is that if you put that in the context of the rates of wetland loss that we've seen over the last century, it doesn't even come close," he added.

The study reports that wetland loss at present is more like 45 square kilometers a year, or more than one acre an hour (an acre is close to the size of a football field).

Louisiana's coastal wetlands are valuable as the home to human communities and also because they help protect New Orleans from hurricanes and sea-level rise. At the same time, they're also a major habitat for birds, and they nourish fisheries that humans, in turn, rely on.

The research was published in Science Advances and led by Tulane's Elizabeth Chamberlain with colleagues at Tulane and other institutions in the United States, Britain, Austria and the Netherlands.

The scientists took their samples in areas of solid land where you would hardly expect there was once open water or a marsh — but then, that's just the point. The Mississippi River is a great builder of land as it carries large volumes of sediment and silt downstream.

Some of that sediment is trapped behind dams along the length of the river, but much still reaches the delta. The problem is that factors that drive wetland loss are simply more powerful — the sinking of the land (subsidence), the intrusion of saltwater as seas rise, the dissolution of wetlands that have been cut into canals to support oil and gas pipes, and more.

And on top of that, sea-level rise is occurring much faster than it did when the Bayou Lafourche land was built.

The current rate is about 3.2 millimeters per year, and it is believed to be accelerating. But when the Lafourche subdelta grew, the sea-level rise rate was just 0.6 millimeters per year.

"Lafourche formed during a relatively favorable time when the rate of sea-level rise was about as low as it can get in this region," Tornqvist said.

The conclusion is that, well, the river just might not be able to keep pace. That's even though the Louisiana Coastal Protection and Restoration Authority, using funds from the BP settlement, is moving forward with two large sediment "diversions" that within a few years could start channeling huge volumes of river water in new directions, in a bid to protect areas around New Orleans in particular. Many scientists have applauded the plan as a way of harnessing nature's power to counter land loss.


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