Scientists raise questions over diversions’ effectiveness
An agreement signed Wednesday by the state and the U.S. Army Corps of Engineers signals an intent to move forward with Mississippi River diversions as a major component of coastal restoration.
That agreement involves cost-sharing for a five-year study of how much water and sediment in the river are available to be diverted for restoration and the most efficient way to use those resources.
Meanwhile, some scientists are questioning just how beneficial these diversions are and if the state and federal coastal representatives are viewing these diversions realistically.
A new report in Geophysical Research Letters says freshwater diversions in Louisiana haven’t produced more vegetation or more land.
In fact, the freshwater diversions have made coastal wetlands in the diversion areas more vulnerable to damage from tropical storms, the report says.
“Ultimately, the scientific basis for river diversions needs to be more convincing before embarking on a strategy that may result in marshes even less able to survive hurricanes,” the report says. “The scientific basis for the benefits for freshwater diversions is not settled.”
The report, “Freshwater River Diversions for Marsh Restoration in Louisiana,” is written by Michael S. Kearney and J.C. Alexis Riter — both with the department of geography at the University of Maryland — and Eugene Turner, with the department of oceanography and coastal sciences at LSU.
The researchers compared satellite photos in the marsh area around three diversions — at Caernarvon, Naomi in Plaquemines Parish and West Pointe a la Hache in Plaquemines Parish — as well as areas outside of the diversions’ influence.
The photos were compared to determine any changes in vegetation or land mass before and after the diversions opened. The study’s focus ended before hurricanes Katrina and Rita in 2005.
In addition, there were field observations to double-check what they were seeing in the photos.
“We have shown that three long-running diversion projects initiated to restore Mississippi Delta coastal marshes failed to increase vegetation cover or overall marsh area,” the report says.
One problem is that nutrients contained in river water that have been washed out from farmland and urban areas upstream can weaken coastal marshes areas, Turner said.
For instance, the nutrients speed up the deterioration of the organic material that makes up the soil.
Because of the nutrients, plants don’t need to grow extensive root structures, which constitute much of the organic soils in the marsh.
With the additional nutrients, plants put more effort in growing above-ground vegetation, Turner said.
Put together, these factors mean that diversions of this nutrient-rich water result in top-heavy, weak-rooted vegetation that is more vulnerable to storms, he said.
The vulnerability of these marshes can be seen in the amount of land destroyed after Hurricane Katrina, Turner said.
He predicted the report will generate criticism because of the number of diversions already operating along the river and the plans for larger and more sediment-focused diversions envisioned for the future.
“Asking questions about this (river diversions) is going to be taken as you’re opposed to restoration,” Turner said. “You appear to be in their way if you ask questions.”
Anticipating possible criticism, Turner said detractors will likely say these diversions were never meant to build land, but only to push back intruding saltwater that moves inland as coastal marshes continue to be lost. Although that may have been the reason for the authorization to build the Caenarvon diversion, the project was sold to the public as something that would build land, Turner said.
So, what should be done for coastal restoration?
Turner’s answer: “We don’t want to make things worse. First, do no harm.”
Caernarvon — on the east bank of the Mississippi near the St. Bernard-Plaquemines parish line — was authorized as a project to help push back saltwater. The Water Resources Development Act of 2007 gave the corps permission to study how the Caernarvon diversion and the Davis Pond diversion, on west bank of St. Charles Parish, two miles below Luling, could be operated differently to put more sediment in the marsh, Miller said.
Work on both of those possible changes is ongoing, said Greg Miller, chief of the plan formulation branch with the corps. “There does appear to be opportunities to move sediment into the marsh,” he said.
Miller said he welcomes the report because it can help inform future planning.
One criticism of the paper is that it examined diversions that are relatively small compared to what needs to be built for land-building purposes.
Don Boesch, professor of marine science and president of the University of Maryland Center for Environmental Science, is one of the scientists named in the paper as supporting diversions. Boesch is a former executive director of the Louisiana Universities Marine Consortium in Cocodrie.
Boesch said that not only is Caernarvon a small diversion, it was designed to deliver fresh water, not sediment, to the marsh.
He also said the problem of the nutrient-rich water is one of the most compelling reasons to put pressure on the U.S. Environmental Protection Agency and upriver states to start making a dent in the amount of this nutrient pollution.
These nutrients are also the cause of the annual summer low-oxygen dead zone in the Gulf of Mexico, he said. By reducing nutrient loads in the river, the issue of any soil weakening from freshwater diversions can also be addressed, he said.
John Day, distinguished professor emeritus of the Department of Coastal and Environmental Sciences at LSU, and another scientist mentioned in the report as having given strong support for river diversions, offer more-strident criticism of the paper.
“The paper doesn’t present any new information, so we don’t learn anything new from it,” Day said. “The reasons they give in the paper are speculative.”
In fact, he said, parts of the wetlands in the Caernarvon diversion outflow are very healthy.
He said it’s not surprising there was marsh damage in the Caernarvon area after the 2005 hurricanes since fresher marshes are more susceptible to hurricane damage. Areas of freshwater marsh that weren’t influenced by river diversions also suffered major damage.
“It (the paper) doesn’t at all prove that diversions don’t work,” Day said.
Day pointed to other areas of the state, specifically in the Atchafalaya River outflow where the Wax Lake delta marsh is growing and includes healthy marsh. As one of the largest “diversions,” it shows that there can be land built if the diversions are large enough, he said.
Most recently, it’s been reported that a cut in the west river bank below Venice at West Bay flowing into a shallow bay has built land after this spring’s high water in the Mississippi.
Day said most of the scientific community has moved beyond any concerns about using diversions from the Mississippi River.
“If we don’t do diversions, what do we do?” Day said. “The model should be what the river does naturally.”