Mississippi River diversion, rebuilding wetlands

Sediment

U.S. Army Corps of Engineers

The opening of the Mississippi River’s Morganza Spillway allowed Douglas Jerolmack, an assistant professor in the Department of Earth and Environmental Science, to lead research into how river diversions might rebuild coastal marshes.

Most scientific experiments are carefully planned and plotted, controlled, and contained. But in a recent study led by geologists at Penn, such was not the case. Instead, the research focused on an event that researchers could not control: the Mississippi River floods of 2011.

The floods—among the most extreme ever recorded on the river—compelled the U.S. Army Corps of Engineers to open the Morganza Spillway for the first time since 1973 to divert some of the Mississippi’s waters into the Atchafalaya River Basin, a swampy area of central Louisiana. Seeing an opportunity, the researchers, led by Douglas Jerolmack, an assistant professor in the Department of Earth and Environmental Science, aimed to compare the movements of the floodwaters and sediment that flowed downriver in the Mississippi Basin to those in the Atchafalaya Basin.

“Since it was a big, slow-moving flood, we could use the Army Corps of Engineers' projections to assess how much time we had before the floodwaters hit the coast,” says Jerolmack. “We assembled a team, wrote a grant proposal, and had the funds to actually start the research right as the floodwaters from the Mississippi arrived on the Delta.”

Helicopter

Funding from the National Science Foundation allowed the researchers to hire a helicopter to collect sediment samples.

Funding from the National Science Foundation allowed the researchers to hire a helicopter to collect sediment samples, and a boat to take water samples and hydrologic measurements in the Gulf of Mexico. The scientists also monitored satellite images of the region. Their investigations revealed that the water and sediment emanating from the mouth of the Mississippi River came out like a fire hose, shooting sediment out to sea. In contrast, the broad plume of sediment-laden water flowing over the Morganza Spillway into the Atchafalaya Basin moved more slowly and deposited more sediment on coastal marshes.

With Louisiana’s wetlands sinking and disappearing under rising seas, these findings have implications for creating modifications to the Mississippi River’s levee structures to help restore the state’s protective marshland.

Jerolmack says scientists, engineers, and managers are all converging on the idea that some diversions from the Mississippi channel can harness some of the sediment that is being spit out the mouth of the Mississippi into the deep ocean, and channel it into areas where it will deposit and build back marsh. 

“For us, this is an important field-scale demonstration that the hydrodynamics of a river matter a lot for how much sedimentation and marsh building occur,” he says. “That’s something that could be applied to the design of artificial rivers and diversions on the Mississippi in the future.”

Originally published on October 25, 2012