NEW ORLEANS — Ducks have taken roost on a sandy strip along the Mississippi River – a bank that's typically underwater.
"We have this nice little beach here that Black-bellied whistling ducks are enjoying," says Heath Jones, chief of emergency management at the New Orleans District of the U.S. Army Corps of Engineers.
Drought has sent water levels plunging to near record lows on the Mississippi River. A river gauge near Corps headquarters registers just 3 feet above sea level.
"It's approaching some historical lows that we've had here," Jones said on Oct. 19 as he looked out from the river levee.
The meandering Mississippi has a cool blue-green hue – different than the muddy current that typically rushes by.
More than a third of the rain in the United States ends up in the Mississippi River system. Jones says with little or no rainfall coming from the Midwest, the drought is causing problems along the river. Ships and barges are running aground, and navigation is slowed up and down the busy shipping corridor.
The salty Gulf is pushing upriver
Here in south Louisiana it's causing a unique phenomenon, changing the point at which the freshwater river and salty sea meet.
"As the flows in the Mississippi River drop, the Gulf of Mexico essentially comes upstream," Jones explains.
A saltwater wedge has crept along the river bottom nearly 64 miles upriver from the mouth of the Mississippi.
"It's almost like a triangle," Jones says. "As this flow in the Mississippi River drops, it loses its ability to keep saltwater at bay."
The saltwater intrusion is threatening both municipal drinking water supplies in the New Orleans metro area and commercial water users like oil refineries that depend on fresh water from the Mississippi.
The biggest impact so far is in Plaquemines Parish with about 24,000 people and water-dependent industries south of New Orleans.
"The Gulf is winning," says councilman Benny Rousselle.
The parish has declared a state of emergency and issued a drinking water advisory.
Rousselle says salt water has already compromised two of the parish's water treatment plants and is threatening a third.
"We're bringing in some desalinization units," he says. "To be able to take the salt out and manufacture water."
Building a submerged levee to stop the saltwater wedge
To save Plaquemines' biggest plant, and protect the larger Orleans Parish water system, the Corps of Engineers is trying to block the saltwater from encroaching farther.
"We are building, for lack of a better term, an underwater levee," Jones says. "We call it a saltwater sill, but essentially it's a big mound of sand, a berm of sand that stops the saltwater."
A contractor pumps the sand from the river bed to create a submerged wall stretching from bank to bank across the Mississippi. The sill is built to allow 55 feet of clearance so big ships can still pass over it.
It's hard to imagine being able to stop water from flowing over a deep dam, but Jones says the dense saltwater stays at the bottom of the water column.
"The Gulf doesn't have the force to push it over the top."
Saltwater comes upriver to some extent every year, but has only threatened water supplies about every ten years. The Corps built similar multi-million-dollar underwater levees in 1988, 1999 and in 2012.
A taste of sea level rise
Some experts say saltwater intrusion could be a more frequent threat now that the Corps is dredging the Mississippi river even deeper for navigation, which allows the saltwater to move in faster. And, then there's climate change.
"You're really tasting sea level rise," says Mark Davis, director of the Tulane Institute on Water Resources Law and Policy.
"The more sea level rises, the more saltwater comes in," Davis says. "We've made it easier this year because we recently dredged the mouth of the river so it would be deeper, so larger cargo vessels can come in. And that just opens the door for even more salt."
For now, the underwater levee will remain in place until the Mississippi River has enough flow to eventually wash it away.
Copyright 2022 NPR. To see more, visit https://www.npr.org.