Louisiana wetlands built up by big floods; levees hold back nourishing sediment

By on March 6, 2013
Doctoral student Nicole Khan used a helicopter to survey Louisiana marshes (Credit: University of Pennsylvania)

Even the darkest clouds can have a silver lining—sometimes it just takes a team of researchers to find it.

After one of the most devastating Mississippi River floods of the past century, University of Pennsylvania researchers descended to Louisiana wetlands in the summer of 2011 to see if among the destruction there might have been positive growth in the area’s marshes.

Led by doctoral student Nicole Khan and advised by Associate Professor Benjamin Horton of the Department of Earth and Environmental Science, researchers sought to study the impact the flood had on sediment composition in the area’s wetlands, which had been sinking from compaction and diminished by rising sea levels.

Maintaining wetlands is crucial for sustaining Louisiana’s ecosystem. Besides providing a habitat for countless species of plants and animals, wetlands also play a role in the region’s water purification and flood control.

Seasonal floods and hurricanes are essential for wetland growth. However, the ramifications of a once-in-a-century flood were uncertain.

“We used the opportunity of this large-scale flood to give us some much needed data for how much sediment is brought down, specifically during one of these large-scale events,” Horton said.  “They don’t happen very often, so when one occurs, you need to use the opportunity at the time.”

Khan and U.S. Geological Survey researchers traveled by helicopter to 45 different sites across four wetland basins, taking five core samples of sediment at each location.

The researchers then compared the core sample measurements to existing sediment accumulation measurements from Coastal Reference Monitoring System sites.

“What we found is that the accumulation during the flood, depending on what basin you were in, was around 40 to 80 percent of what you’d see deposited during a normal year,” Khan said.

Khan and a U.S. Geological Survey collaborator measure a sediment sample from the wetland.

Khan and a U.S. Geological Survey collaborator measure a sediment sample from the wetland.

Although the deposits to the Louisiana wetlands were substantial, how the accumulation varied between basins was unique. Substantially more sediment deposited in the Atchafalaya River Basin than the Mississippi River Basin, even though the Mississippi River had much more water displaced.

The researchers concluded that the levees used to control the Mississippi River flood waters were also acting to restrain sediment essential for the wetlands’ growth.

“The Atchafalaya deposited larger amounts of sediments than the Mississippi even though the Mississippi has far more water flowing down it,” Horton said. “That’s because the Mississippi is very efficient in taking sediment offshore where the Atchafalaya, because there are no levees there, deposits sediment on the wetlands.”

The resulting data could lead to measures to help modify existing levees to help curb flood waters, while at the same time, allow for increased wetland deposits.

“One idea is to basically punch holes into the levees so that during spring floods, they’re now delivering all that sediment to the marshes instead of keeping it in the channel and shunting it off to the Gulf of Mexico,” Khan said.

Besides helping researchers gain a better understanding of sediment transference in the Louisiana marshes, the collected data may also help researchers understand the sediment itself more completely.

Through additional examination, researchers found a potential marker in the soil that may help them better understand the relationship between flooding and wetland preservation.

Flood water sediment had a higher ratio of round diatoms, or photosynthetic algae, to rod-shaped diatoms. Since round diatoms are more likely to be found in the water column during a flood, researchers believe that their presence in sediment could be an indication of both recent and historical floods.  This information could be instrumental in helping researchers understand past floods and determine if they might be linked to climate or human modifications of the landscape.

“We think we have an indicator for past floods,” Khan said. “So, we can look for the changes in the sediment to look at the frequency of past floods, and then look at the time scale before and after human modification of the system to see if it changes the frequency or the magnitude of the types of sediment being deposited during large floods.”

Top image: Doctoral student Nicole Khan used a helicopter to survey Louisiana marshes (Credit: University of Pennsylvania)

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