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Katrina, Rita Actually Helped Wetlands, Study Says

Willie Drye
for National Geographic News
September 21, 2006
 
A new study makes the provocative claim that Hurricanes Katrina and Rita
actually helped stabilize coastal wetlands by depositing tons of silt
and sediment—even as the storms devastated dozens of square miles
of the low-lying areas.

The new findings contradict long-held theories that rivers are the primary source of the sediment that forms wetlands, says research leader R. Eugene Turner, an ecologist at Louisiana State University (LSU) in Baton Rouge.

The study also counters beliefs that the loss of wetlands—especially on the eastern Louisiana coastline—has been caused by flood-prevention levees on the Mississippi River, he adds.

Coastal wetlands are breeding grounds for many marine animals. They also protect coastlines from hurricane sea swells, or storm surges.

Though sediments are a relatively small fraction of a wetland, such deposits are an important part of the physical framework supporting wetland plants.

In findings that could cause a stir among environmentalists, the LSU researchers ultimately conclude that hurricanes play an important role in maintaining the health of wetlands.

"I don't think most people expected that," said Mark Ford, deputy director of the Coalition to Restore Coastal Louisiana (CRCL) in Baton Rouge. "It does sound a little counterintuitive.

"I expect it to stir up a lot of conversation. But that's good."

The study, which was funded by a $25,000 grant from the National Science Foundation, appears today in the online edition of the journal Science.

Sediment Surprise

Restoring Louisiana's diminished wetlands has become a major political issue in the state.

The U.S. Geological Survey estimates that Louisiana lost at least 100 square miles (260 square kilometers) of coastal wetlands because of Katrina and Rita. (Related: "Many Islands 'Gone,' Wetlands Gutted After Katrina, Experts Say" [September 2005].)

Turner and researchers Joseph J. Baustian, Erick M. Swenson, and Jennifer S. Spicer conducted their study in November 2005, soon after Katrina and Rita struck (Hurricane Katrina and Hurricane Rita complete coverage.)

Katrina made landfall on August 29, 2005, on Louisiana's eastern coast near the mouth of the Mississippi River (Louisiana map). The storm later made a second landfall on the Mississippi coast (Mississippi map). Hurricane Rita came ashore September 24, 2005, near the Louisiana-Texas border.

Both hurricanes pushed large storm surges onto shore. The surges left behind sand, silt, and dissolved clay.

The research team traveled west from Louisiana's eastern border with Mississippi to a few miles into Texas, collecting 198 sediment samples.

The samples revealed that the hurricanes had dumped from 1 to 6 inches (3 to 15 centimeters) of new sediment in the Louisiana wetlands—about 144 million tons (131 million metric tons) of new sediment in total, study co-author Baustian says.

"We now have a better appreciation for how much sediment can be added by hurricanes," Baustian said.

Engineers have tried to increase sediment deposits by diverting the flow of the Mississippi River through coastal wetlands. But the LSU researchers say the two hurricanes last summer brought in 227 times more sediment than the river diversions bring in a year.

"The diversions we have now introduce a trivial amount of sediment compared to hurricanes," Turner said.

New Approach to Wetlands Loss

The study authors say their findings mandate a new approach to understanding the loss of wetlands and how to reduce that loss.

They say that the loss of plants and other "organics" could be doing more to diminish wetlands than previously realized.

The plants' roots hold the wetlands in place, Turner says. If the plants are lost, the wetlands lose their stability.

CRCL's Ford says that the role hurricanes play in helping wetlands has not been extensively studied.

But he thinks ecologists and environmentalists will raise questions about the LSU study's conclusions, including whether the estimates for the amount of sediment moved by last summer's storms are too high.

"Turner is very well respected," Ford said. "But not everyone agrees with him."

Still, the LSU researchers' work could prompt more studies of how hurricanes may benefit wetlands, Ford says.

Willie Drye is the author of Storm of the Century: The Labor Day Hurricane of 1935, published by National Geographic Books.

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