Army Corps builds a temporary barrier to block salt water encroaching on the nation’s largest river.

Army Corps of Engineers raising the height of an underwater barrier, or sill, across the bed of the Mississippi River, in southern Louisiana, in September 2023. The Corps is again building the temporary sill this year, in an effort to keep salt water from the Gulf of Mexico from moving upriver. Photo courtesy of the Army Corps of Engineers/Ryan Labadens

By Brett Walton, Circle of Blue – September 10, 2024

Communities in Plaquemines Parish, at the mouth of the Mississippi River in southern Louisiana, are on alert yet again as salt water threatens to spoil their drinking water supply for a third consecutive year.

Salt water is moving upriver from the Gulf of Mexico. Drought in the central United States has weakened the mighty river’s flow to the point that it can no longer push out saltier water from the Gulf. For Plaquemines Parish, which draws directly from the river, this intrusion is a threat to its water source.

As a defensive measure the Army Corps of Engineers is now building a temporary barrier on the river bed to impede the salt water. The Corps is also staging equipment in case it needs to barge in fresh water, like it did last year when it moved 153 million gallons to areas in southern Louisiana where saltwater intrusion was a problem.

Because of actions taken last year – including interconnections with neighboring parishes that allow for water sharing – Plaquemines officials say they are better prepared for saltwater intrusion.

As of September 6, the leading edge of the saltwater “wedge” was 45 miles upstream of the river mouth. The barrier, made of dredged sediment that will erode when river flows strengthen, will be built 64 miles from the mouth.

This is the third consecutive year and sixth time overall that the Corps has built the barrier.

“It’s very unusual to have three years in a row where we get flows this low,” said David Welch, a hydrologist with the Lower Mississippi River Forecast Center.

Last year, saltwater moved so far inland – some 69 miles – that the Corps had to raise the barrier. Doing so protected New Orleans’s drinking water intakes, but it did not help Plaquemines Parish, where water treatment plants in Boothville, Port Sulphur, and Pointe à la Hache were affected. All three are now installing reverse osmosis systems to remove salt.

Salt water and drinking water systems do not mix. To begin with, salts can corrode pipes and other equipment. High chloride concentrations are also a health concern, especially for people with high blood pressure or kidney disease, and for infants and pregnant women. The EPA has a non-enforceable standard for chloride of 250 parts per million, which is set primarily for its unpalatable taste.

Saltwater intrusion in the river is a matter of physics. Starting just south of Natchez, Mississippi – some 348 miles upriver from the mouth – the bed of the Mississippi is below sea level. Because it’s denser than fresh water, the saltwater wedge sinks and moves along the river bottom. Water flows downhill, so the Gulf of Mexico, if it were unimpeded, has a long runway inland.

Most of the time, the force of the Mississippi, which drains 40 percent of the Lower 48 states, is strong enough to repel the Gulf. But not at the moment.

Construction of the temporary barrier could begin this week, according to Matt Roe, an Army Corps spokesperson. Completing the underwater wall that tops out at 45 feet in height should take about two weeks, he said. It will span the river but still allow access for commercial shipping, which is also disrupted when the river drops.

Three consecutive low flow events is unusual, but will a weaker Mississippi be more common on a warmer planet with higher seas? The federal government’s most recent climate assessment notes that while overall precipitation in the basin is likely to increase, summer precipitation – when the Mississippi is usually its lowest – will be more variable. Combined with higher temperatures, that increases the risk of flash droughts that develop rapidly. A flash drought in 2022 contributed to record low water levels on the Mississippi and saltwater intrusion.

Southern Louisiana today is in the path of Tropical Storm Francine, which is expected to reach hurricane status when it makes landfall by Wednesday evening. The National Weather Service warns of flash floods in the area.

The relatively quick burst of rain, however, will not solve the saltwater problem.

“Any help we do get will be short-lived,” Welch said.

Most of the Mississippi’s flow comes from the Ohio River, its main tributary. That area needs moisture, Welch said.

“As dry as it’s been over the Ohio, a lot of the rain is going to just soak up into the ground,” Welch said. “It’s not going to generate a lot of runoff that’s going to really bring river levels up.”