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MELISSA BLOCK, HOST:

This is ALL THINGS CONSIDERED from NPR News. I'm Melissa Block.

AUDIE CORNISH, HOST:

And I'm Audie Cornish. The drought that's gripping much of the country is taking a toll on the Mississippi River. Less water is flowing and river traffic has been brought to a standstill just south of Memphis. Further south, at the mouth of the Mississippi, low outflow means saltwater from the Gulf of Mexico is creeping in. That's threatening the drinking water supply for the New Orleans area, as we hear from NPR's Debbie Elliott.

DEBBIE ELLIOTT, BYLINE: Traffic is still moving in the lower Mississippi, including a ferry that runs commuters between New Orleans and Algiers.

AARON BURROW: If you don't own a car, that's the only way to get across the river.

ELLIOTT: Aaron Burrow works on the water. He's a chef on a riverboat that's currently docked upriver because of the drought. He knows how the low flow affects navigation, but didn't consider what it could do to the water supply.

BURROW: Drinking water's always been fine. The Mississippi River and the communities have been living along here a long time.

ELLIOTT: The saltwater has already moved 90 miles upriver and is just five miles from the landing where he got on the ferry. Saltwater typically encroaches up the Mississippi every eight to 10 years, but that could be changing, according to Mark Davis, director of the Tulane Institute on Water Resources, Law and Policy.

MARK DAVIS: With rising seas and if we talk about deepening the channel 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, you know, the vitality of industries and the health of populations.

ELLIOTT: Industries and communities that tap the river for fresh water. 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 moves in.

DAVIS: Saltwater hugs the bottom of the river because it's heavier and it's denser than the fresh water 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 work its way toward New Orleans.

ELLIOTT: It's already affecting neighboring Plaquemines Parish to the south. Parish president, Billy Nungesser, no stranger to disaster, declared a state of emergency a couple of weeks ago when redfish started showing up at the river and saltwater breached parish water intakes.

BILLY NUNGESSER: You know, we've been through four hurricanes and an oil spill and, if you'd told me this was going to happen, I wouldn't have believed you.

ELLIOTT: The parish is now buying water piped in from New Orleans and other systems and is also sending barges upriver to gather fresh water.

NUNGESSER: And we are drawing water from those barges. The parts per million of salt got well over 1,000, so it required us to do that to give good drinking water to the people of the parish.

ELLIOTT: 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 last few days and New Orleans district commander, Colonel Ed Fleming, says the Corps is working to stop it.

ED FLEMING: To try and arrest the development of the saltwater coming further north, we built a sill in the bottom of the river.

ELLIOTT: It's basically an underwater barrier to block the saltwater wedge.

FLEMING: The sill is an earthen levy built on the bottom of the river, on the floor of the river. We don't want to impact commercial traffic that comes up and down the river.

ELLIOTT: The Corps used a similar strategy in 1988 and '99 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.

New Orleans Sewerage and Water Board director Marcia St. Martin.

MARCIA ST. MARTIN: 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.

ELLIOTT: A matter of taste because health officials say the saltwater is not a danger to most people, unless they're on a low sodium diet. Debbie Elliott, NPR News.

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