The Next Disaster

What would happen if the Old River Control Structure at the head of the Atchafalaya River were to collapse?

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Louisiana’s Atchafalaya River is unique. A distributary of the Mississippi, it flows out of the Big Muddy near Simmesport and runs approximately 125 miles to enter the Gulf of Mexico near Morgan City. Looking at the broad, turbulent river today, it is hard to imagine that at times during the early nineteenth century, people could actually cross it on a fifteen-foot plank.

The Atchafalaya begins in the Three Rivers region where the Mississippi, Red, and Atchafalaya come together. About two hundred years ago, the Red ran into the Mississippi at Turnbull’s Bend, and the Atchafalaya ran out of the Mississippi a little farther downstream. A twenty-mile-long log jam at this juncture acted as a dam that allowed little water to trickle down the Atchafalaya’s channel.

In 1831, Henry Miller Shreve cut a new channel across the narrow neck of Turnbull’s Bend to straighten out the Mississippi so steamboats would have a shorter route into the Red. As part of the “improvements,” he also cleaned out the raft that blocked the Atchafalaya. Removing the logs was like pulling a plug, and the Red suddenly began discharging all of its water down the Atchafalaya instead of the Mississippi. In no time, the minor stream was scoured out and became the wide river we know today. Before the log jam was cleared out, much of what is now the Atchafalaya Basin was actually a huge lake called Grand Lake. When the modern-day Atchafalaya River was created with the clearing of the raft, the sediment-laden water would overflow the bank during floods. Gradually, the backswamp and Grand Lake was filled in and became dry land.

[Read: Gravel grinding on the Atchafalaya Wilderness Bike Trail.]

Shreve’s “improvements” also set in motion a chain of events that could one day devastate Louisiana. The Atchafalaya’s route to the Gulf is shorter than the Mississippi’s, and it has a steeper grade. Because water seeks the path of least resistance, the Mississippi began trying to change course and flow down the Atchafalaya soon after Shreve removed the log jam. It came dangerously close to doing so during the great 1927 flood.

To prevent this from happening, the federal government completed the huge Old River Control Structure near Simmesport in 1963. It diverts all of the Red River and about one-third of the Mississippi down the Atchafalaya while keeping the mighty Mississippi in its channel. During floods, control gates can be opened to allow more Mississippi water to flow into the Atchafalaya and relieve pressure on the Mississippi levees downstream.

So far, the Old River Control Structure has worked as planned, but the massive lock and dam was in danger of collapsing during the 1973 flood. If that ever happens, the Mississippi River will immediately divert down the Atchafalaya, and the consequences will make our modern hurricanes, oil spill, and floods seem like minor inconveniences. It is estimated that about seventy percent of the Mississippi’s water would flow through the Atchafalaya Basin, weakening or perhaps even collapsing the highway and railroad bridges that cross it. With the loss of U.S. Highway 90 and I-10, east-west traffic would be completely disrupted, creating a traveler’s nightmare and higher transportation costs.

The raging water would probably force the abandonment of Morgan City and other basin communities, and the oil and gas wells, pipelines, and canals would be destroyed. In a replay of the BP disaster, the oil spillage would be immense, and it might take years to repair the damage. Gasoline prices would soar, commercial crawfishing and recreational fishing would be dramatically impacted, and the increased sediment and fresh water would ruin the area’s oyster beds and shrimp industry.       

Such a structural collapse would also cause the lower Mississippi to shrink by two-thirds almost overnight. Constant dredging could probably keep ocean-going vessels moving, but shipping might be interrupted during times of low water.            

A more serious problem would be the effect a smaller Mississippi River would have on Baton Rouge, New Orleans, and the ecosystem. Because the river’s current would be greatly reduced, the Gulf’s saltwater would intrude far upstream, killing the vegetation along the river bank and causing soil erosion. Eventually, the river below Baton Rouge would widen to become a giant saltwater estuary.

Expensive levee systems might protect New Orleans and smaller towns from eroding, but the communities and industries from Baton Rouge to the Gulf would lose their source of water. Underground aquifers would not be able to fill the need, and multi-billion dollar pipelines would have to be constructed to bring in water.

The implications of an Old River Control Structure collapse are so enormous that it is difficult to wrap one’s mind around it. But after Hurricanes Katrina and Rita, the BP oil spill, and the floods of 2016, Louisianans have learned that such unimaginable disasters can take place—often in rapid succession. Hopefully, strong measures will be taken to keep the Mississippi River out of the Atchafalaya, but it’s hard to stop Mother Nature.

Sources

• U.S. Army Corps of Engineers' website on the Old River Control Structure: mvn.usace.army.mil/Portals/56/docs/PAO/Brochures/OldRiverControlBrochure.pdf

• Roadside Geology of Louisiana by Darwin Spearing.