New modeling is deepening understanding of why saltwater intrusion up the Mississippi River has worsened in recent years, pointing to a previously underestimated factor as the primary cause and raising questions over how drinking water can be protected in the future.

The study of river dynamics by Tulane University researchers shows that crevasses, or breaks in the lower Mississippi’s banks, are the main factor in the recent worsening of saltwater intrusion, which can threaten drinking water for vast areas of southeast Louisiana, including New Orleans.

Sea level rise and the deepening of the river for shipping purposes have also contributed, but the crevasses located in Plaquemines Parish south of where the main levee system ends are by far the primary cause because of the way they slow the current in the main channel, the study shows.

The findings are in line with preliminary assessments from the Army Corps of Engineers, which is engaged in its own three-year study of the phenomenon, but dealing with the problem poses a series of difficult questions related to coastal land loss, shipping and infrastructure.

The deepening of the lower river through large-scale dredging to accommodate bigger vessels has long been acknowledged as a factor in exacerbating saltwater intrusion, and that has spurred assumptions that those projects have been the main cause of the recent worsening. The new modeling disputes that.

“Deepening should not be considered as the main issue here,” said Ahmed Khalifa, the lead author of the new study published in the peer-reviewed journal Estuarine, Coastal and Shelf Science.

“Definitely we showed that it has some part … but not as much as, for example, sea level rise or closing any of the passes.”

Yet closing all the crevasses, or passes, is not seen as a viable option, and the Tulane research team, led by Ehab Meselhe, is also examining potential nature-based solutions.

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They have presented their findings to the Corps, whose data was employed in the modeling. The Corps is taking an extensive look on its own, even building a physical model of the lower river at its Vicksburg, Mississippi, research center, said spokesman Ricky Boyett.

The research is vital as the region grapples with how to ensure safe drinking water and avoid the corrosion of infrastructure well into the future. Climate change-related effects, particularly sea level rise and increased drought in the Mississippi River basin, are expected to become greater factors in the future.

Complex threat

The threat is a complex one that occurs seasonally, when river flows drop in the summer and fall. That allows salt water to move upriver from the Gulf in the shape of a doorstop, its leading edge crawling along the river bottom because it is heavier.

The Corps has deepened the lower river over the years to accommodate the increasing size of oceangoing vessels, with the most recent project completed in 2022 and bringing it to a depth of 50 feet.

Because deepening worsens saltwater intrusion, the Corps developed a plan to mitigate it, constructing an underwater levee, or sill, along the river bottom to slow the salt water’s advance when it projects that it will be necessary.

The sill had initially been required about once a decade, but it has now had to be built four years in a row. In 2023, initial projections showed the salt water reaching New Orleans, setting off an emergency declaration from the White House and a scramble for a temporary solution to protect drinking water and related infrastructure.

The salt wedge eventually retreated before reaching New Orleans as high flows returned to the river later that year, but it served as a wake-up call for local officials. New Orleans Mayor Helena Moreno has raised the possibility of building a regional water treatment plant as one potential solution, but that comes with extremely high costs and would require multi-parish coordination.

The crevasses are largely the result of natural occurrences, with the Mississippi breaking its banks in a number of spots downriver, similar to how it formerly behaved before the construction of the vast levee system that keeps it in place. The slower flow of the main channel speeds the advance of the salt intrusion.

The most prominent crevasse in recent years is called Neptune Pass, which at one point expanded so much it was diverting up to 18% of the river into a nearby bay and wetlands. The Corps recently constructed a rock barrier to partially block Neptune, which is located across the river from Buras, bringing the flow down to around 8%.

It is not yet clear what effect the reduction of Neptune Pass flow will have on saltwater intrusion since the Tulane modeling used 2023 conditions as a base. Other crevasses included in the modeling are also located along the river’s east bank: Mardi Gras Pass, Bohemia Spillway, Ostrica Pass and Fort St. Phillip.

While shipping interests have long called for the closure of the crevasses because of the complications they create for vessels, the passes are also acting as natural river diversions, creating land the way the Mississippi used to do. That has led coastal advocates to argue for keeping them at least partially open.

Closing them could also simply lead to more crevasses being created elsewhere since the river will eventually find its level.

The main focus of the $5.5 million Corps’ study, which is in its first year, is on whether its current mitigation plans involving the sill remain sufficient to deal with the effects of the deepening, said Boyett. But the research could also lead to other types of projects that address the problem more generally.

The Tulane researchers note that the future effects of sea level rise and other climate-related changes will be important. Boyett said the Corps is incorporating future conditions such as sea level rise in its modeling.

“We’re looking at it to, first, determine what’s causing it, if we are appropriately mitigating for it and — if we are and it’s still occurring — what are the other factors and how would one go about addressing that?” said Boyett.

“We don’t necessarily say we’re going to have all the answers at the end of the modeling. But it may be that modeling gives us recommendations for pilot projects that we should try out.”

‘What is the potential?’

The details of the Tulane modeling are revealing.

It showed that, in 2023, sea level rise over the past three decades caused salt intrusion to advance about 4 additional miles, while the most recent channel deepening led to a 2.2-mile difference.

As for the east bank crevasses, the modeling showed that if all of them were closed — widely seen as impossible, but instructive for study purposes — the salt water would have advanced 75 fewer miles. Closing only Neptune Pass and Fort St. Philip resulted in a 47-mile difference, the modeling showed.

The study also looked at whether moving the sill to another location would improve its effectiveness in slowing the salt water’s advance. It concluded that moving it farther downriver would indeed work better.

Further, the study determined a river level threshold for when the New Orleans area would face a severe threat. It found that a sustained flow of 125,000 cubic feet per second in the lower Mississippi would result in salt water overtopping the sill and advancing to New Orleans. That level is an extreme low, but it does occur occasionally.

One potential solution being examined by the Tulane team as well as the Corps is whether building sand dams in front of the crevasses when the flow is low could help. The idea could potentially satisfy lots of needs since the sand would wash away once river flows increase, building land farther downstream in the passes.

A number of studies are ongoing related to the lower river and the many challenges it presents, particularly as conditions evolve under a warming climate.

The Tulane team is part of a larger project on the future of the river’s mouth, known as MissDelta and financed by the National Academy of Sciences. A separate look at how to deal with the crevasses is being studied by the Baton Rouge-based Water Institute, the University of New Orleans, Nunez Community College in Chalmette and the California Institute of Technology.

In addition to the Corps’ three-year study of salt intrusion, it is also carrying out a five-year “megastudy” of the entire lower river and how to manage it, from Cape Girardeau, Missouri, to the Gulf.

“We also have to look at it from a perspective of, ‘will there be more crevasses in the future? What is the potential?’” Boyett said of the Corps’ salt intrusion study.

“We’re doing it to look to make sure we’re doing what we’re supposed to be doing. But we also need to better understand what’s happening to the river down there.”