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A rising Mississippi River bottom could change the river's course in a future flood

An aerial view of the Auxiliary Structure at the Old River Control Complex (John Snell) An aerial view of the Auxiliary Structure at the Old River Control Complex (John Snell)
(WVUE) -

The Mississippi River wants to break free of its levees and plot a shorter route to the Gulf of Mexico, bypassing New Orleans in favor of present-day Morgan City.

The Old River Control Structure is designed to resist the river's urges by sending just enough water down the down a man-made channel toward the Atchafalaya River.

By law, 70 percent of the flow moves south through New Orleans, the balance down the Atchafalaya. 

"If the structures were not built and all, the river would eventually change course to that of the Atchafalaya River," said Russell Beauvais, Operations Manager for the U.S. Army Corps of Engineers.

Through the centuries, the river has shifted course as it searches for a shorter, steeper route to the Gulf.

"So, it just meanders through South Louisiana trying to find the easiest course to get there," Beauvais said.

The path down the Atchafalaya would cut roughly 100 miles off the current river's course.

"The purpose of the Old River complex is to essentially stop time in 1950," said Will Veatch, Senior Hydrologist for the Corps New Orleans District.

In 1950, worried the river was about to blow through its bank and dart toward Morgan City, congress authorized the Corps to build the structure. A series of flood gates unleash water into the man-made channel, connecting the Mississippi with the Red and Atchafalaya Rivers. In the spring, it can be the closest thing to Louisiana white water rapids.

During the flood of 2011, nearly 700,000 cubic feet per second poured through Old River, enough to fill almost eight Olympic-size pools per second. 
"It goes from being standstill to where you could have rapids coming out of the structure with waves 6 feet tall or higher," Beauvais said.

In December, an LSU researcher warned a future mega flood could overwhelm the structure and cause the river to alter its course toward down the Atchafalaya.

Hydrologist Yi-Jun Xu presented his findings at a meeting of the American Geophysical Union in New Orleans.

"It will cause widespread flooding in the Atchafalaya (river basin) and Morgan City will probably be gone, covered with water," Xu told NOLA.com.

Sediment from the Mississippi also would fill the present Atchafalaya Bay within 30 years, he said.

Veatch and other government hydrologists say they are aware of changes in the river bottom, but they believe the issue has not approached anything urgent.

From Vicksburg to Baton Rouge, the Corps says a giant slug of sediment on the river bottom is steadily working its way south.

No one knows how much, but Veatch said it amounts to "Superdomes and Superdomes" worth of sand and mud.

Veatch described "tens of millions of cubic yards of material that's slowly working its way down the Mississippi River over the course of decades or even centuries." 

As it builds up, the sediment raises the river bottom.

It also raises the possibility - the Corps says very remote - that in a large flood, an immense amount of water would swell the river and compromise the structure.  

"We don't think that there's a high probability of a failure today," Veatch said.

However, he concedes as the sediment collects over time, the chances increase that it would create bigger issues.

"It's a low probability, but high consequence kind of situation," Veatch said.

The consequences would be huge. Depending on how dramatically the switch happened, first Baton Rouge, then New Orleans would lack the water to serve ocean-going ships. It could jeopardize billions of dollars invested in chemical plants, threaten drinking water supplies with salt moving up the old channel, and potentially wipe out Morgan City.

Over time, the great river in New Orleans would come to resemble Bayou Lafourche in Thibodaux, one of its ancient paths.  

It may all seem a bit far fetched, but it actually almost happened in 1973 in the great spring flood. With water building up around the original structure, a wing wall was collapsing as the river scoured two holes under the structure.   

"That caused the Corps immediately to commandeer all the rock on the river, up and down in Mississippi River, and start dumping into large scour hole," Beauvais said.

That rock remains in place today along the original Old River Control Structure.

Today the "structure" is actually a set of giant gates, spaced over several miles: the original structure that nearly failed; the newer, auxiliary structure congress authorized after that near miss; and a closure and lock farther downriver complete the complex.

"One thing we do have is a high degree of control over the flow in the river," Veatch said.

The Corps believes it has a number of tools at its disposal to combat potential problems, including dredging the river bottom, building dykes in the river to alter the sediment flow, and operating the Bonnet Carre and Morganza Spillways downriver. 

The Old River Control Structure could also contribute to changes in the sediment flow and might need to be operated differently at some point in the future, Veatch said. 

Most likely, Veatch said the long term solution would involve a combination of the tools in the Corps tool box.

"We want to be careful that we're picking an option that doesn't solve one problem and create another."

In the aftermath of the 2011 flood, the Corps already had started an assessment of changes in the Mississippi River, which includes the operation of the Old River Control Structure.

For nearly seven decades, the Corps has succeeded in stopping time, safely keeping the river in its straight jacket of levees.

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