Studies Look At Louisiana’s Coast, Delta

Louisiana’s Mississippi River Delta and coast have made headlines of late, thanks to a pair of studies that examine flooding and coastal erosion, a state-issued coastal quarantine and the recently-announced accelerated permitting for a linchpin project in Louisiana’s master plan to restore and preserve its coast.

The first study, conducted by the Woods Hole Oceanographic Institution (WHOI), a private marine science research center based in Woods Hole, Mass., seeks to connect the frequency and severity of flooding on the Mississippi River to efforts over the last century or more to channelize and control the river.

“The floods that we’ve had over the last century are bigger than anything we’ve seen in the last 500 years,” Sam Muñoz, lead author for the study, said in a statement announcing the research. “There’s been a longstanding question about the extent to which all the changes we have made to the Mississippi River—one of the most engineered rivers in the world—have altered the probability of really large floods.”

The WHOI study group analyzed core samples extracted from three oxbow lakes along the Mississippi River—False River and Lake Saint John in Louisiana and Lake Mary in Mississippi. Scientists involved in the study were able to examine sand grains from layers extracted from the oxbow lakes to determine the size and dates of floods dating well before the Great Flood of 1927. Using a variety of techniques, the scientists claim to have mapped out the flood history of the Mississippi River dating back some 500 years.

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The research, published in the journal Nature, found that over the past century and a half, the frequency of 100-year flood events has increased some 20 percent. The research team placed about three quarters of the blame on modifications to the Mississippi River itself and its basin. The team also identified a connection between floods on the Mississippi River and oceanic weather cycles, like El Niño. Certain climate cycles affecting ocean surface temperatures lead to increased rainfall within the Mississippi River Basin.

“We’re able for the first time to really parse out how the natural variability of the climate system influences flooding, and then how people have modified that,” Muñoz said in the announcement.

The WHOI research team hopes to apply the same methodology to other river systems around the world to identify the causes and effects of river flooding elsewhere.

Channelization Effects

A second study, published in the journal Marine Geology, found that the impact of controlling the Mississippi River extends offshore as well. The article, titled “Mississippi River Subaqueous Delta Is Entering A Stage Of Retrogradation” argues that channelization and the reduction of sediment loads in the river have led, not only to subsidence in Southeast Louisiana, but also to the erosion of the seafloor of the Gulf of Mexico.

“Imagine this as an underwater extension of land loss that we see at the surface,” said Jillian Maloney, lead author for the study and an assistant professor at San Diego State University. “This is a big deal because it can affect so many processes that occur from the coast to the open ocean, including marine organisms’ lifecycles and underwater landslides.”

The study, which drew on nautical charts and historical data from sources like the oil and gas industry and the National Oceanic and Atmospheric Administration, examined mapping data that spanned more than 200 years. The study estimates that, due in part to flood control efforts over the past century, the amount of sediment the Mississippi River carries toward the Gulf of Mexico has been cut in half. That reduction has caused the Mississippi River’s subaqueous delta, the seafloor south of the Louisiana coast, in other words, to erode.

“From this comprehensive study, we’ve determined that the Mississippi River Delta has entered a stage of decline,” said coauthor Sam Bentley, a professor in the Louisiana State University Department of Geology and Geophysics. “The outlets of the Mississippi River, also known as the Bird’s Foot Delta, have been prograding, or spreading, naturally for hundreds of years, but that has now stopped. The underwater portions of the delta are now retreating like the land loss occurring in our landscape.”

Bentley said he suspects other major river deltas around the world could be experiencing a similar decline.

Bug Attacks Delta

As it turns out, humans aren’t the only creatures threatening the Mississippi River Delta in Louisiana—an invasive species of scale is too. And the threat is so serious that the Louisiana Department of Agriculture and Forestry has declared an emergency quarantine of Roseau cane in Louisiana, the predominant vegetation that makes up the state’s coastal marsh.

A scale native to Asia was introduced to the South Louisiana Roseau cane somewhat recently, and the insect is decimating Roseau cane, which is key to holding together the delta.

“Following an extensive survey conducted by the LSU AgCenter, it has been determined that efforts should be made to limit the spread of Roseau cane scale,” said Mike Strain, commissioner of Louisiana’s Department of Agriculture and Forestry. “Therefore, a quarantine is now in effect.”

In all, 26 parishes (counties in Louisiana) are included in the quarantine, which prohibits the moving or transplanting of Roseau cane.

State officials fear the mass die-off of Roseau cane could rapidly transform portions of the Louisiana coast from thick marsh to open water, further exacerbating the state’s coastal erosion problem. The state also fears the invasive scale could eventually attack the state’s sugarcane crop.

No course of action to eradicate the bug has been identified as yet.

Sediment Diversion

The news isn’t all bad, though, for the delta in South Louisiana, thanks to an agreement between the State of Louisiana, the U.S. Army Corps of Engineers, and other federal agencies to shave years off the permitting process for the state’s planned Mid-Barataria Sediment Diversion.

Louisiana’s Coastal Protection and Restoration Authority (CPRA) announced April 2 that the Corps of Engineers had moved up the project’s anticipated permitting date nearly two years, from October 2022 to November 2020.

In a statement praising the Corps’ action, CPRA Chairman Johnny Bradberry made clear the significance of the project to the state’s fight to save its coast.

“The Mid-Barataria Sediment Diversion is critical to our future, as it addresses the root cause of our coastal crisis by reconnecting the Mississippi River with our basins and restoring the natural process that built our delta,” Bradberry said.

The planned diversion will be located near Ironton, La., in the parish of Plaquemines near river Mile 61 and will divert up to 75,000 cubic feet per second of water and sediment to the Barataria Basin on the west bank of the Mississippi River. Current estimates place the cost of the project at about $1.4 billion. The project is in the pre-construction phase, with funding coming from the Deepwater Horizon oil spill criminal settlement. CPRA hopes construction costs will be covered by Deepwater Horizon Natural Resource Damage funds.

The state also has plans to build a smaller sediment diversion on the east bank of the river to nourish Breton Sound.

According to CPRA, the Barataria and Breton basins and the Mississippi River’s Birdfoot Delta have lost an estimated 700 square miles of land since the levees were constructed in the 1930s.