A new study released this week confirms what coastal restoration scientists have thought all along: The slow-moving Atchafalaya River does a better job of building wetlands during major floods than the Mississippi River, which moves at a much faster pace and dumps most of its sediment into the deep waters of the Gulf of Mexico. That's good news, considering several restoration projects on the drawing board would attempt to recreate the Mississippi's original flooding patterns before it was hemmed in by an elaborate man-made levee system.
Birds fly over the Morganza Floodway after it opened May 14, 2011, to lower the height of the Mississippi River and the potential flooding of Baton Rouge and New Orleans.
The study, which was published Sunday as a peer-reviewed letter in the online edition of Nature Geoscience, used satellite data and ground sampling to measure the effects of last year's record-breaking flood, when the Army Corps of Engineers opened the Morganza Floodway to divert water from the rain-swollen Mississippi River into the Atchafalaya River Basin. It was only the second time in 60 years the floodway was opened.
The study's results confirm that the state's plans to build diversions along the Mississippi River's banks, using designs that would slow the movement of floodwaters into Barataria Bay and Breton Sound, will build land, said lead author Douglas Jerolmack, a sediment dynamics researcher at the University of Pennsylvania.
During last year’s flood, the river levees that stretch from Baton Rouge to just south of Venice created a fast-moving jet of sediment-rich water that punched through coastal currents that might have otherwise kept the sediment near the shoreline, according to the study.
MODIS Aqua visible image of sediment-rich waters exiting the Atchafalaya River, left, and Mississippi River, right.
While only about a third of the Mississippi’s sediment content was transferred to the Atchafalaya with the opening of the Morganza Spillway, the water carrying it moved much more slowly as it reached the Gulf.
That allowed the sediment to be captured by coastal currents and spread over a large area, creating a thicker layer of soil.
A more detailed paper explaining the research will be published in Geology magazine later this year, Jerolmack said.
The study was conducted by three teams of researchers representing five universities in the United States and Italy, the U.S. Geological Survey and Italy's National Research Council. One team led by recently retired USGS research ecologist Karen McKee used helicopters to measure the depth of new sediment at 45 locations along Louisiana's coast.
“There was a very clear layer of soil deposited on the surface, with a different color,” McKee said. “The sediment contained organisms that told us about its source, that it came from the river, as opposed to the marine environment. Another key was the lack of plant roots, which told us this was a very recent deposit.”
A satellite image shows the concentration of suspended sediment (ssc) in 2011 floodwaters is greater at the mouth of the Atchafalaya River than at the birdsfoot delta of the Mississippi River. Circles show deposits of sediment in the Atchafalaya Delta area are greater than in the birdsfoot delta. Source: NASA MODIS Aqua satellite data, calibrated with field data. b: Sediment accumulated in greater amounts and over a wider area in the Atchafalaya Delta area. There was significant accumulation in the immediate area of the Mississippi's Southwest Pass, and little to no deposition between the two deltas.
The team compared its sampling to similar data collected routinely by the federal-state Coastwide Reference Monitoring System, which tracks coastal changes related to wetlands restoration efforts.
“We found that the more diffuse sediment plume associated with the Atchafalaya and Wax Lake outlets was getting into the marshes, that there was more sediment associated with that area compared to the Mississippi River’s outlets,” she said.
A third team from the Mississippi Mineral Resources Institute at the University of Mississippi was measuring the amount of sediment in water exiting the Mississippi River.
The Mississippi team’s research ship was directed to sampling locations by Jerolmack and his students, who used the measurements to calibrate images of the floodwaters flowing from the river captured by NASA’s MODIS Aqua satellite.
“They scooped up a sample and measured its sediment content,” Jerolmack said, and the number would correspond to the color intensity on the satellite photograph, producing a map estimating the amount of sediment in water along the coast.
The jet flowing from Mississippi’s Southwest Pass, the largest of the three channels at the river’s mouth, was clearly visible on the satellite images measuring the water’s sediment content, and on separate images tracking the river’s colder freshwater as it flowed into the warm Gulf of Mexico.
But the jet also was felt by researchers, as it was moving fast enough to disrupt their sampling, Jerolmack said. Even 18 ½ miles offshore, the water from Southwest Pass was moving at close to 3 ½ miles per hour, enough to make floating in one place difficult.
By combining the three sets of information, the scientists were able to map the dynamic relationship between the coast’s geography and the amount of sediment being deposited.
At the Mississippi’s Bird’s Foot Delta – basically the region around the river’s mouth -- the sediment measured in the water was significant, but most was carried offshore.
Sediment is visible in West Bay, where a diversion from Southwest Pass slows the water flow. But elsewhere in the delta, the sediment levels are low, compared to the area at the mouth of the Atchafalaya and to the west.
There, sediment-rich floodwaters flowed slowly out of several channels at the mouth of the Atchafalaya, including the man-made Wax Lake Outlet, mirroring the way the Mississippi’s historic deltas used to distribute water and sediment along the coast.
Between the Bird’s Foot Delta and the Atchafalaya, however, the amount of sediment deposited by the flood was minimal, and little sediment was seen in the satellite images.
In July, a similar study in Nature Geoscience by researchers with the University of Illinois and the Army Corps of Engineers found that last year’s floodwaters deposited significant quantities of sand grains in the Bonnet Carre Spillway, which acts like the historic mouth of the Mississippi when it is opened to allow floodwaters to enter Lake Pontchartrain.
Combined, the two studies provide clear evidence that properly designed diversions that mirror the river’s historic distribution system will create land efficiently, Jerolmack said.
“We hope we can convince the scientific community, and more importantly, the engineering community, that we understand this enough now to do it efficiently.”
The sediment study was funded by grants from the National Science Foundation, the National Oceanic and Atmospheric Administration, the European Commission, and the University of Pennsylvania’s Benjamin Franklin Fellowship.