OXFORD, Miss. – Levees along the Mississippi River do much to prevent devastating floods in riverside communities, but they have also contributed to the loss of Louisiana’s wetlands. A new study offers insight into how new diversions in the Mississippi River’s levees may help restore Louisiana’s wetlands.
University of Mississippi researchers Carol B. Lutken, Leonardo Macelloni and Marco D’Emidio joined a team of geologists and others who used the Mississippi River flood in spring 2011 to document the fate of sediment delivered by floodwaters to the Mississippi Birdsfoot Delta. The team’s article, “Linking the historic 2011 Mississippi River flood to coastal wetland sedimentation,” appears in the November issue of Nature Geoscience and its website.
“By holding in floodwaters, the levees prevent sediment delivery onto much of the river’s natural floodplain,” said Lutken, associate director of the Mississippi Mineral Resources Institute at UM. “They focus greater volumes of water over a diminished area, which results in more of the flow arriving at the river’s mouth. This focusing of flow also enables the river to carry a greater suspended load of sediment and increases the proportion that will bypass coastal marshes.”
Sediment that could contribute to the marshes, which provide protection from storm surges and hurricanes, is bypassing the coastal areas and being carried into the Gulf of Mexico. Without sufficient sediment replenishment, marshes in the northern Gulf are being lost to sea-level rise and damage from coastal storms.
“Scientists, engineers and local governments discuss and sometimes propose ways of adjusting the levee system to allow floodwaters to access areas of deteriorating marshlands in hopes of restoring some of the natural wetland-building ability of the Mississippi,” Lutken said. “This project is the first that we know of that includes in situ hydrographic measurements during a large-scale flood event. We hope to use this study to learn more about how river diversions can be used to promote marsh-building and to provide a resource for policymakers in coastal communities.”
The 2011 floods broke records across several states, damaged homes and crops and took several lives. The predicted destruction was reduced, however, because U.S. Army Corps of Engineers opened the Morganza Spillway, a river-control structure, for the first time since 1973 to divert water from the Mississippi into the Atchafalaya River Basin, emulating a natural flood.
This action involved the deliberate flooding of more than 12,000 square kilometers, alleviating pressures on downstream levees and sparing Baton Rouge and New Orleans from the worst of the flood.
“The significance of our research is tremendous because it greatly impacts the general public, especially the people living in the Gulf region,” said Macelloni, MMRI research associate. “First, the field measurements conducted both in the marsh and at the river mouth after this flood are the first ones to be conducted using modern scientific techniques. Our data is a treasury for future studies.
“Second, it shows clearly that this critical event, although very disruptive, is important for marsh preservation. Third, the Mississippi River seems to have a natural tendency to deposit the sediments off-shore, bypassing the coastal area. We have to understand why and what part the human changes in the river play in its present so we can act responsibly in the future.”
The group calculated the amount of sediment in the plumes for the duration of the flood based on the ocean color in satellite images. They calibrated these data to field samples taken in the Gulf of Mexico by the UM team during the flood. Satellite-guided boat sampling allowed them to collect water and sediment data from several outflows of the river, to gather data on the speed of the plume and the extent to which river water mixed with ocean water.
This field component was designed and executed by the trio with marine systems specialists Matt Lowe and Brian Noakes, who design MMRI’s sampling equipment. Real-time satellite and target location updates arrived every four hours from Federico Falcini, project director at the University of Pennsylvania.
“Satellite images showed that the Mississippi River effluent at Southwest Pass penetrated the coastal current, carrying tremendous volumes of sediment many kilometers out into the Gulf of Mexico,” said D’Emidio, navigation and GIS expert at MMRI. “The waters diverted into the unleveed Atchafalaya Basin spread out over many thousands of square kilometers of land and around 100 kilometers of coastline, making it more likely that the sediment would remain in the marsh.”
U.S. Geological Survey researchers used a helicopter to travel to 45 sites across the Atchafalaya, Terrebonne, Barataria and Mississippi Birdfoot basin wetlands, where they collected sediment cores. They found 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 revealed another signature of new sediments: they contained a higher proportion of round diatoms to rod-shaped diatoms than did deeper layers of sediment, and are typical of fresh water, pointing to deposition from suspended sediment from rivers.
“Taken together, the team’s 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,” Macelloni said.
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 marshes in other areas of the Delta.
The work was supported by the National Science Foundation, National Oceanic and Atmospheric Administration, Luquillo Critical Zone Laboratory, European Commission and the University of Pennsylvania’s Benjamin Franklin Fellowship. The study was headed by Douglas Jerolmack, assistant professor in the Department of Earth and Environmental Science at the University of Pennsylvania, and Falcini, who at the time was a postdoctoral researcher in Jerolmack’s lab and is now at the Consiglio Nazionale delle Ricerche in Rome.
Other Penn researchers involved included Benjamin Horton, associate professor in the earth and environmental science department, Nicole Khan, a doctoral student in Horton’s lab, and Alessandro Salusti, a visiting undergraduate researcher. The Penn researchers worked with Rosalia Santoleri, Simone Colella and Gianluca Volpe of the Consiglio Nazionale delle Ricerche, Karen L. McKee of the U.S. Geological Survey and Chunyan Li of Louisiana State University.