All the dry weather means there's less water flowing through the once mighty river into the Gulf of Mexico, and low outflow means saltwater from the Gulf is creeping in.
Some Louisiana cities have already begun purchasing drinking water. Now New Orleans is at risk.
Saltwater typically encroaches on the Mississippi every eight to 10 years, but that could be changing, says Mark Davis, director of the Tulane Institute on Water Resources, Law and Policy. "With rising seas, and if we talk about deepening the channels of the rivers to accommodate bigger ships, this kind of thing is something you may face more frequently, and if you get it wrong, it can affect the vitality of industries and the health of populations," Davis says.
Normally, the strong outward flow of the Mississippi keeps saltwater at bay. But Davis says the lower river bottom is below sea level, so when there's less flow coming downstream, the Gulf wedges its way in.
"Saltwater hugs the bottom of the river because it's heavier, denser than the freshwater. And that's what makes it a wedge, not a wall. And right now, given the low flows in the Mississippi and the level of the Gulf, we're watching the water make its way toward New Orleans," Davis says.
It's already affecting neighboring Plaquemines Parish, to the south. Parish President Billy Nungesser declared a state of emergency a couple of weeks ago when redfish started showing up in the river and saltwater breached parish water intakes.
"We've been through four hurricanes and an oil spill and if you'd have told me this was going to happen, I wouldn't have believed you," Nungesser says.
The parish is now buying water piped in from New Orleans and other systems and is also sending barges upriver to gather fresh water. "And we are drawing water from those barges. The parts per million of salt got well over a thousand so it required us to do that to give good drinking water to the people of the parish," he says.
The U.S. Army Corps of Engineers says the saltwater has been encroaching more than a mile a day, but that pace has significantly slowed in the past few days. And New Orleans District Commander Col. Ed Fleming says the corps is working to stop it.
"To try and arrest the development of the saltwater coming further north, we built a sill in the bottom of the river," Fleming says.
It's basically an underwater barrier to block the saltwater wedge.
"The sill is an earthen levee built on the bottom of the river," Fleming says. "We don't want to impact commercial traffic that comes up and down the river."
The corps used a similar strategy in 1988 and 1999 to protect the New Orleans water supply and officials are optimistic it will work again. And the problem goes away once the rains return upriver.
"At the moment I don't think it's a threat. We do not anticipate having to experience saltwater as a matter of taste coming into the plant," says Marcia St. Martin, the New Orleans sewerage and water board director.
A matter of taste because health officials say the saltwater is not a danger to most people unless they are on a low-sodium diet.