As a wedge of saltwater creeps up the Mississippi River, the U.S. Army Corps of Engineers is considering an unusual tactic: Using the massive Old River Control Structure to reduce the amount of water flowing from the Mississippi into the Red River and the Atchafalaya Basin. 

Due to a drought-starved, low-flow river, salt water from the Gulf of Mexico is moving up the Mississippi and now threatens drinking water for more than 1 million people in the New Orleans area. The saltwater wedge is expected to overwhelm Belle Chasse’s, New Orleans’, Gretna’s and Jefferson Parish’s drinking water intakes and have its deepwater leading edge reach the Luling area by the end of October, according to current Corps’ estimates.

Reducing the amount of diversion at Old River would mean more water in the Mississippi, which could help flush the salt back downriver — or so the argument goes.

When the threat first arose weeks ago, Corps officials said they found the shift would have created a whole new set of salt intrusion problems for other drinking water systems in southwestern Louisiana that depend on the Atchafalaya. And they said it could set in motion problems for wildlife and navigation along the Atchafalaya River.

And, they argued, the change would produce little benefit for the Mississippi’s saltwater problem.

But now, water flows in the Red River are rising, an impending rain front in the could dump even more water into the Red River Valley. So the Corps of Engineers is taking a second look at the idea, agency officials said on Monday.

“As we see the contributions of the Red River increase, we will keep evaluating the opportunities to identify benefits, if any, Old River may have on the saltwater intrusion,” Ricky Boyett, a spokesman for the Corps in New Orleans.

The reevaluation will look again at negative impacts in the Atchafalaya Basin and whether the change in water distribution would have an appreciable benefit on the Mississippi.

Boyett cautioned that any proposal to pursue the idea would need review from regional Corps officials beyond the New Orleans district.

“Any insight we gain shall be shared with the Mississippi Valley Division to identify the best path forward that yields the least damaging approach that yields the greatest benefits,” he said.

How it would work

The Old River Control Structure keeps the Mississippi from following Mother Nature’s pent-up desires and changing its course to head down the Atchafalaya River, away from Baton Rouge and New Orleans.

The nearly 60-year-old series of locks, dams, channels and guide levees is located at a key inflection point among the Red, Atchafalaya and Mississippi rivers near the notch in Louisiana’s boot north of Morganza.

The Red empties entirely into and becomes the Atchafalaya just west of the Old River Control Structure. The structure ensures the combined flow of the Mississippi and Red upriver of the structure remains at a 70/30 split in the Mississippi and the Atchafalaya below the control structure.

The structure does this only by diverting water from the normally much bigger Mississippi into the Red and Atchafalaya and does not operate in reverse, Corps officials said.

Faced with temporary pipelines and freshwater barges to keep drinking water available, several readers of The Advocate|The Times-Picayune have been asking why the Corps of Engineers can’t temporarily alter the Old River split and keep more water for the Mississippi to push back the salt wedge.

“Could the flow be temporarily increased during the Intrusion?” one Metairie reader asked Sunday. “We need to think outside the box.”

Boyett said the Corps has faced similar questions in recent weeks as the salt wedge was moving north. He said the potential negative impacts were significant, officials found, when the Red and Atchafalaya were so low a few weeks back.

The Corps’ analysis had found a salt wedge in the Atchafalaya would have affected drinking water for Morgan City, Berwick and Patterson.

The reduced flow in the Atchafalaya would also have created ecological impacts, such as changes in temperature, oxygen and salinity. They would affect protected areas like Henderson Swamp and the Atchafalaya and Sherburne wildlife management areas.

In addition, navigation limits on the Atchafalaya would have been affected.  Bayou Teche and the Vermilion River could face problems as well, because they rely on water pumped from the Atchafalaya.

When the Corps produced this earlier analysis, flow in the Red was so low that diversion from the Mississippi was contributing almost 94% of the flow in the Atchafalaya below Old River.

“This wasn’t an option a few weeks ago because the Red River was so low (changing diversion) would have basically dried up the Atchafalaya,” Boyett said.

But the Red’s flow has increased by more than four times since then, rising from 3,000 cubic feet per second to 17,000 cfs.

Under the normal operations of Old River, increased flow from the Red naturally means less diversion from the Mississippi to maintain the 70/30 split between the Mississippi and the Atchafalaya, Boyett said.

That means more water is already being held in the Mississippi than had been occurring a few weeks ago. The Corps’ current task, Boyett said, will be to evaluate what kind of benefit holding additional water in the Mississippi would provide.