Study: Bank Failures Reduce Mississippi Flow

Floods have caused openings on the east bank of the Lower Mississippi River below New Orleans that are reducing the river’s power, scientists say.

This issue was explored April 14 in the webinar “Implications of Water Loss from the Lowermost Mississippi River.” America’s Watershed Initiative, a nonpartisan nonprofit organization seeking to work with business, government, academic and civic organizations to drive investment and advance science and education concerning the Mississippi River and its tributaries, hosted the event.

Dr. Mead Allison, a professor in the Department of River-Coastal Science and Engineering at Tulane University in New Orleans, was the keynote speaker.

Allison said that, according to recent surveys, water loss from openings along the east bank of the Mississippi River has increased by about 25 percent since 2004.

The largest of these, Neptune Pass, is located 39 miles above the Head of Passes and roughly 70 miles south of New Orleans. It widened during high-water conditions in 2016, 2019 and 2020 and now carries about 14 percent of the river, Alison said. When measured, it was flowing at 156,000 cubic feet/second, making it the third largest river in the United States in terms of discharge, he said. Last year, the Corps of Engineers placed about 330,000 tons of stone to reduce the pass’s inlet from 32,000 square feet down to 10,300 square feet in an attempt to reduce the flow to the amount before the 2019 season.

While Neptune Pass is the largest, other openings are also significant. Among them is Mardi Gras Pass at Mile 43.7 above Head of Passes, which developed following 2011 flooding. A study showed water flowing out of that pass at 21,000 cubic feet per second. An April 2025 survey in the Fort St. Philip reach, about 11 miles above Head of Passes, found several unintentional breaches leading to the loss of water at 128,000 cubic feet per second.

“What’s happening here is with the absence of a levee and rising sea levels and subsidence, we’re starting to see failures occur during large floods along this section of the bank between River Mile 44 and Head of Passes,” Allison said.

What to do about these breaches can be controversial, Allison said. Some environmentalists note that the sediment diversion caused by these channels is creating marshland at a time when Louisiana’s coastline is disappearing. However, the sudden influx of freshwater is threatening to destroy or has destroyed high-salinity oyster beds and, along with them, commercial fishermen’s livelihoods.

Allison focused his comments primarily on the potential for river navigation impacts, increased dredging costs and risk to drinking water in south Louisiana communities.

He began by noting that this reach of the river is extremely important to the navigation industry with the five ports of south Louisiana comprising a 54-mile port district frequently top ranked in the nation in terms of total tonnage passing through it, including agriculture exports. The Port of New Orleans is the main port handling container cargo, with $1 billion in assets, he said.

The Mississippi River has been deepened to 50 feet south of Baton Rouge, meaning this last roughly 250 miles of the river is also extremely busy with all kinds of commercial traffic, from deep-draft vessels to towboats and barges and cruise vessels.

“We would argue that these failures and expansion of channels are because of the fact that there is only a natural levee along this section of the river that’s much degraded by a rising sea level, and the absence of an artificial river levee means we are beginning to see a series of failures along this bank line that run the risk of not completely losing the channel but reducing the stream power sufficiently that it’s going to have implications for the deep-draft navigation channel,” Allison said.

Maintaining the river’s navigability is of top importance to the American economy, and regular dredging is required, Allison said. The Southwest Pass dredging is the nation’s most expensive, with about 13.1 million cubic yards dredged annually.

He argued that the water loss from all of these breaches is slowing down the river enough so that it deposits more sediment along the bottom, leading to an increased need for dredging, in some cases in areas that have never needed it before.

“When you reduce the stream power, you reduce the capacity to transport sediment,” he said.

Additionally, during periods of low water, a saltwater wedge creeps along the river bottom from the Gulf up the Mississippi since the bottom of the river is below sea level in that region and the reduced flow of freshwater in a drought allows the saltwater to creep upriver. While the Corps of Engineers builds a sill across the river bottom to prevent this creep from impacting the municipal drinking water of some more populous areas during these low-water conditions, many downriver communities are affected, with drinking water needing to be trucked in on some past occasions. Allison said the reduced river flow from the breaks is likely to make that saltwater intrusion more frequent.

Allison added that one recent study modeled the predicted effect on saltwater intrusion with both Neptune Pass and the large exits in the Fort St. Philip reach closed and found the saltwater intrusion was predicted to stay 15 to 30 miles farther downriver, depending on the river’s flow rate.

Allison said the Corps of Engineers currently has no mandate to close these exits, but that given the studies completed so far, “I think there’s going to be a policy change that is going to have to be made.”