Mississippi River Delta to "Drown" by 2100?

Rebecca Carroll
for National Geographic News
June 29, 2009

The Mississippi River Delta is drowning, according to new research that predicts the surrounding coastline will be inevitably reshaped in coming decades.

"There's just not enough sediment to sustain the delta plain," said study author Michael Blum of Louisiana State University in Baton Rouge.

Deltas are coastal landmasses created from a river's sediment deposits as the water flows out to sea.

The Mississippi River's delta plain, for example, includes the lacy "toe" of southern Louisiana. (See a Louisiana map.)

All deltas are degrading to some extent, as their sediment settles and sinks. But a delta can sustain itself or even grow if its parent river regularly deposits enough new material.

Today sediments collected along the Mississippi cover about 23,360 square miles (60,500 square kilometers) ranging in thickness from less than 33 feet (10 meters) upstream near Memphis, Tennessee, to about 328 feet (100 meters) in the delta at the tip of southern Louisiana.

The drainage basin of the roughly 2,350-mile-long (3,782-kilometer-long) river, however, includes about 40,000 dams and levees built over the past century.

These structures control flooding and improve navigation, but they also trap sediment or funnel it completely through to the sea.

Previous studies suggested that dams and reservoirs built since 1950 have trapped as much as 70 percent of the river's natural amount of sediment. With less material feeding it, the delta plain has been experiencing erosion.

(Read a National Geographic magazine article about erosion in Louisiana that seemed to "predict" the damage done by Hurricane Katrina.)

But even without the dams and levees, the amount of sediment flowing downriver would no longer be enough to sustain the delta because of rising seas, the study authors say.

Tough Choices

The researchers base their conclusions on estimated delta levels over the past 12,000 years, which show significant changes more than 7,000 years ago, when meltwater from the last ice age quickly filled the oceans.

The Mississippi Delta plain retreated inland at that point, and it was only after sea level rise had slowed considerably that the delta again grew seaward.

Current sea level rise, however, may be three times faster than it was the last time the delta was able to grow. (Related: "New York, Boston 'Directly in Path' of Sea Level Rise.")

With the added threat of rapid sea-level rise, sustaining the current extent of the delta plain would require 18 to 24 billion tons of sediment—way more than the entire Mississippi River currently carries, the researchers say.

The team therefore estimates that as much as 5,200 square miles (13,500 square kilometers) of delta land could disappear by 2100—an area only slightly smaller than Connecticut.

For now the study authors don't have a solution, and they add that plans to save the delta plain—such as redirecting and possibly adding sediment—will almost certainly involve sacrifices.

"They can [divert sediment to areas] downstream from, say, New Orleans, but that means that areas [of the delta plain] farther upstream will be submerged," Blum said.

"Tough choices have to be made, and they need to be made fast."

Findings appear online this week in the journal Nature Geoscience.




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