A Mississippi river diversion helped build Louisiana wetlands
also allowed them to gather data on the speed of the plume and the extent to which river water mixed with ocean water.
From the satellite images, researchers observed that the Mississippi River unleashed a jet of water into the ocean. In contrast, the waters diverted into the Atchafalaya Basin spread out over 100 kilometers of coastline, the sediment lingering in a wide swampy area.
“You have this intentionally flooded Atchafalaya Basin and when those flood waters hit the coast they were trapped there for a month, where tides and waves could bring them back on shore,” Jerolmack said. “Whereas in the Mississippi channel, where all the waters were totally leveed, you could see from satellite images this sort of fire hose of water that pushed the sediment from the river far off shore.”
The release notes that the researchers used a helicopter to travel to forty sites across the two basins, where they sampled sediment cores. They observed that sediment deposited to a greater extent in the Atchafalaya Basin than in any area of the Mississippi Basin wetlands, even though the Mississippi River plume contained more total sediment.
The recently deposited sediments lacked plant roots and were different in color and consistency from the older sediments. Laboratory analyses of diatoms, or photosynthetic algae, also revealed another signature of newly deposited sediments: They contained a higher proportion of round diatoms to rod-shaped diatoms than did deeper layers of sediment.
“This diatom ratio can now serve as an indicator for freshwater floods,” Horton said. “With longer sediment cores and analyses of the diatoms, we may be able to work out how many floods have occurred, how much sediment they deposited and what their recurrence intervals were.”
Taken together, the researchers’ findings offer a large-scale demonstration of how flooding over the Atchafalaya’s wide basin built up sediment in wetland areas, compared to the more-focused plume of water from the Mississippi River. Jerolmack says this “natural experiment” provides a convincing and reliable way of gathering data and information about how changes in the Mississippi’s levees and control structures could help restore marsh in other areas of the Delta.
“One of the things that we found here is that the Atchafalaya, which is this wide, slow plume, actually produced a lot of sedimentation over a broad area,” Jerolmack said. “We think that what the Atchafalaya is showing us on a field scale is that this is the sort of diversion that you would need in order to create effective sedimentation and marsh building.”
— Read more in Federico Falcini et al., “Linking the historic 2011 Mississippi River flood to coastal wetland sedimentation,” Nature Geoscience (21 October 2012) (doi:10.1038/ngeo1615)
