Sediment carried by 2011 Mississippi flood shored up Louisiana’s wetlands
in forty-five different sites across four wetland basins and took five cores of sediment at each site (see a video produced by the U.S. Geological Survey depicting the sediment survey).
Just by looking at the cores, the geologists could tell what portion was deposited from that season’s flood.
“The flood sediments had a different color and a looser consistency than the older sediments,” Khan said. “The recent sediments also lacked plant roots, because plants hadn’t had time to colonize and put roots in the deposits since the flood.”
Comparing measurements from the cores to records of sediment accumulation collected at nearby Coastal Reference Monitoring System sites, the researchers calculated how deposition from the 2011 flood compared to the average yearly sediment deposition. Across the four basins, the flood-carried sediment alone accounted for more than half-a-year’s sediment accumulation. In the Atchafalaya Basin, however, the figure was more striking: The flood accounted for 85 percent, or nearly a year’s worth of sediment accumulation.
The release notes that the team, taking the sediment cores back to the lab, performed additional analyses, looking at grain size and the amount of organic matter compared to mineral matter it contained. These values helped further separate the recent flood sediments from the underlying sediments.
Then the researchers examined the biological properties of the samples by identifying the diatoms, or photosynthetic algae, contained in different layers of the sediment. They discovered that the flood sediments had higher ratios of centric, or round diatoms, to pennate, or rod-shaped diatoms, than the older sediments.
The prevalence of round diatoms in flood sediments makes sense, as these species tend to float in the water column and would likely have been derived from the flowing river water that inundated the wetlands during the flood.
This ratio, the researchers believe, may serve as a reliable signature of both recent and historic floods — a useful indicator if geologists wish to examine deeper sediments for evidence of floods that occurred years or even centuries ago.
“If we can find an indicator in the modern sediments, we can say how common these flood events are,” Horton said. “For example, Hurricane Sandy has been talked about as the storm of a century, but we need to get better evidence to understand the recurrence intervals of large but rare events like Sandy and how they might be related to climate or to land-use changes.”
The researchers hope their findings concerning sediment deposition and their discovery of a flood indicator will motivate future research on how sediment acts to build and maintain wetlands.
“This sets the stage to go back to our sample sites and see whether these marshes maintain their elevations
over time,” Khan said. “We’ll be able to determine if the flood actually helped in terms of having a lasting effect on the wetlands.”
The study was supported by the National Science Foundation, National Oceanic and Atmospheric Administration and University of Pennsylvania’s Benjamin Franklin Fellowship.
— Read more in Nicole S. Khan et al., “Tracking sedimentation from the historic A.D. 2011 Mississippi River flood in the deltaic wetlands of Louisiana, USA,” Geology (7 February 2013) (doi: 10.1130/G33805.1); and Federico Falcini et al., “Linking the historic 2011 Mississippi River flood to coastal wetland sedimentation,” Nature Geoscience 5 (21 October 2012): 803–7 (doi:10.1038/ngeo1615)
