Geo-Joint: Movin’ the Mighty Mississip’

Rivers flow where gravity pulls them, and they effortlessly seek the easiest path. In steep mountain canyons, they tumble down sharply defined channels—there’s no doubt of their course. But when the land is flatter, it’s a good deal easier to influence that fluvial path. As a river meanders through territory of low relief, its course may wander back and forth in tight loops, called oxbows, shaped like the loops that tie the oxen to their yoke. When the oxbow loops get very tight and flooding occurs, the necks of those loops may get overtopped, and the flow will cut off the crescent-shaped lobe as the river shortens its path to the sea. Slow continuous gouging on the outside bank of each curve will eventually do the same trick. The cut-off remainder is called—what else—an oxbow lake. River courses will also shift and jump along braided courses as sediment builds up in one channel and forces water to a now-deeper path. This kind of course change, or avulsion, occurs along the Rio Grande and constantly changes the definition of the US/Mexico border, as recent Geo-Joint readers will recall.
For the most part, a river jumping its banks to forge a new path is a matter of quite local, if any, concern. However, there is a looming situation that may command the attention of millions, and will be noted by people far from the vicinity. The Mississippi River is a major geographical feature. Its course would seem to be so massive that other than occasional flooding and oxbow cutting, it would keep to its path. But the Mississippi has changed its course many, many times in its history and in fact does so about once a millennium. We may get to see one of those epic shifts soon.

The meandering Red River in Arkansas, and some of the oxbow lakes now separated from its flow.

The Mississippi sends the Atchafalaya River some of its water—in the future it might send it all.

People have been attempting to engineer the Mississippi for a long time. Well, they actually have engineered it, but ultimately the river decides how well the plans of man suit its whims. The Mississippi is lined with levees through all of its lower reaches, and for over 3,000 miles overall, with the aim of keeping it in one place. The first efforts at this were in the early 18th century, around New Orleans. The efforts have been met with varying degrees of success, but they don’t write blues songs about levees breaking for nothing. Hurricane Katrina tested those New Orleans levees, and found their weaknesses, with tragic results. Paradoxically, even successful levees bring problems. By channelizing the river, floodwaters are not allowed to carry sediment to the wetlands bordering the river. These wetlands actually subside over time due to the very weight of their wet soils, and unless incoming sediment slowly builds up their elevation, they become quite depressed. When a levee does fail, or when Gulf storms send a push of seawater landward, water does reach these subsided areas, they can be completely submerged, destroying the fragile wetlands environment.

The history of the Mississippi/Atchafalaya connection, and the Old River Control Structure.

Another strategy for controlling the river has been to send in the Army Corps of Engineers to redirect a portion of the flow. The Corps has a long history with the Mississippi, with mixed results. They applied this redirection strategy below Vidalia, Louisiana, by building a project called the Old River Control Structure. It features three gates that shunt more than a quarter of the Mississippi’s flow to a channel that leads it to the Atchafalaya River. From there it runs in a curvy fashion to the west of the Mississippi and roughly parallel to it, down to the Gulf of Mexico at Atchafalaya Bay. In times of flood, this relieves a great deal of pressure off the Mississippi, and saves Baton Rouge, New Orleans and many smaller towns along the way from watery disasters. The Mississippi was no stranger to the Atchafalalya, and would head off in that direction on its own in times of flood, but the fear was that it would do it in a permanent manner, so the Army Corps was put to work.

Levees keep the river in check, until they don’t.

The project has worked pretty well since it was finished back in the early 1960s (and upgraded in the ’80s), but it has been stressed by various floods, and pushed pretty close to its limits. Some feel it will eventually fail, and at least one, Y. Jun Xu of the School of Renewable Natural Resources at LSU, thinks it might only take only one good megaflood to do it. Unfortunately, he says, circumstances are in place that will exacerbate the effect of a big flood. They are caused by the buildup of sediment in areas just downstream of the Control Structure. This deposition has made the channel more narrow, by up to 2,500 feet in one place, and raised the riverbed level elsewhere about 30 feet in the last 20 years. With these new constrictions on flow, waters will back up and pool, creating more pressure on the gates, and possibly overtopping them. With enough flow, the barriers could be undercut and washed out, with the Mississippi forging a new main route.

With a big enough flood, the Old River Control Structure could be washed away. Previous floods have threatened, and the Mississippi River flow rate is expected to rise by 10 to 60 percent by 2100. Prospects don’t look good.

 

Should that happen, there will be disasters in two directions. First, of course, the environment along the Atchafalaya would be inundated and swept away by a flow much larger than it is used to. Many towns would be lost and farmlands permanently submerged by the Mississippi’s new direction. In addition, the sediment carried by the Mississippi would likely fill in Atachafalaya Bay in a few short decades. There’s a reason they call it the Big Muddy. An entirely separate set of calamities would happen along the river’s old course. The diminished flow past Baton Rouge and New Orleans would rob those cities of drinking water, as well as create a crisis for the many petrochemical factories along the route that need water to operate. Shipping goods from the Gulf up the old course to ports inland would become impossible in the shallower waters. And as the flow of fresh water weakened, the Gulf of Mexico’s waters would push north across the very flat delta, making the river brackish, and contaminating vital freshwater aquifers with salt. In time, the river course below the point of detour would become a bayou. The commerce and general function of the whole former lower Mississippi would figuratively and pretty literally dry up.
All is not lost yet, however, and there are things that could be done to help avert the crisis. Dredging up the accumulating sand deposits from choked areas both north and south of the Control Structure and piping it all south to the delta could actually help two problems at once. Clearing out the channel would keep flood waters moving, and increasing the sediment out on the delta would slow the ocean erosion that has endangered towns at the far south end of Louisiana. Another way to clear out the sediment would be to set off explosive charges in it during strong river flows in order to suspend it within the water column and get it moving downstream. These possible solutions are not slam-dunks, and they would be costly. Pushing sediment farther downstream doesn’t always work smoothly, and could cause flooding elsewhere. The risk might be worth it though, because seeing that rare event of a Mississippi River re-route could catastrophically alter an important part of the country.
The situation is a classic example of what happens when humans try to control nature. We may have our way for some time, but usually at considerable financial and environmental expense. Without constant effort, however, and sometimes even in spite of it, nature will do as it pleases, and in the end we must learn to live with it.

Get the spacebird’s-eye view of the Atchafalaya, the Mississippi, and its delta on this beautiful wall map available from Maps.com. Click here for details:

CLICK HERE TO VIEW THE MAP!


PHOTO CREDITS:

caption: The meandering Red River in Arkansas, and some of the oxbow lakes now separated from its flow.
source: Wikimedia Commons: USDA/FSA (Public domain)
caption: The Mississippi sends the Atchafalaya River some of its water—in the future it might send it all.
source: Army Corps of Engineers (Public domain)
caption: The history of the Mississippi/Atchafalaya connection, and the Old River Control Structure.
source: Wikimedia Commons: US Army Corps of Engineers (Public domain)
caption: Levees keep the river in check, until they don’t.
source: Wikimedia Commons: Louisiana GOHSEP (CC by SA 2.0 Generic)
caption: With a big enough flood, the Old River Control Structure could be washed away. Previous floods have threatened, and the Mississippi River flow rate is expected to rise by 10 to 60 percent by 2100. Prospects don’t look good.
source: Wikimedia Commons: Michael Maples, US Army Corps of Engineers (Public domain)

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