Why America Won't Let the Mississippi River Move

In the spring of 1973, a concrete structure in the Louisiana swamps began to shake.

The Mississippi River was in a huge flood, and all that water was pouring through the gates of the Old River Control Structure, about 300 river miles above the Gulf of Mexico. The flood tore into the ground around and beneath it. A wall alongside the structure collapsed. For a few frightening weeks, engineers worried that the whole thing would fail.

If it had, the Mississippi River might have left its channel for good. It would have abandoned Baton Rouge and New Orleans and taken a new path to the sea.

The structure held. But the question it was built to answer is still there: why does America's biggest river want to move, and what does it cost to hold it still? Let's work through three hard questions.

Hard question 1Why does the Mississippi want to move at all?

Guess first: A river reaches a fork. One path to the sea is 300 miles long; the other is 140. Which way does the water want to go?

The short one, of course. And that's the Mississippi's problem.

A river carries mud and sand, and near its mouth it drops that sediment. Over centuries, the river builds land out into the sea: a delta. But every mile of new land makes the river's path to the sea a mile longer. Its slope gets gentler, it flows more slowly, and it drops even more sediment in its own channel. Eventually, a flood breaks through to a shorter, steeper route, and the river jumps. Then it starts building a brand-new delta somewhere else.

The Mississippi has done this again and again. Over the last several thousand years, it has switched its main path roughly every thousand years or so, and southern Louisiana is the patchwork of old deltas it left behind.

In the 1900s, the river was getting ready to jump again. At a place called Old River, a channel connects the Mississippi to the Atchafalaya River, whose route to the Gulf is less than half as long. Let's see what that does to the slope:

Twice the slope means faster water. Faster water scours its channel deeper. A deeper channel captures more of the flow, which flows faster still. It's a feedback loop.

The numbers show it happening. The Atchafalaya took less than 10% of the Mississippi's flow in 1850 and about 30% by 1950. In 1953, the US Army Corps of Engineers concluded that without control, the Mississippi could switch to the Atchafalaya by about 1990.

That would have left Baton Rouge and New Orleans on a dwindling, salty side channel, with their ports, refineries and drinking water in trouble. So the Corps built the Old River Control Structure, completed in 1963. It holds the flow at about 70% down the Mississippi and 30% down the Atchafalaya, the split the river had when the structure was built.

The takeaway: rivers build their own deltas until the path gets too long, then they jump to a shorter one. The Mississippi's next jump is the Atchafalaya, and one set of gates is holding it back.

Hard question 2If holding the river protects Louisiana, why is Louisiana disappearing?

This is the counterintuitive one. The levees and gates keep the river where it is, protecting cities and towns. And yet coastal Louisiana has lost more than 2,000 square miles of land since the 1930s, an area about the size of Delaware. In recent years, roughly a football field of wetland turns into open water about every 100 minutes.

Guess first: Delta land naturally sinks as its soft mud settles. So what kept the delta above water for thousands of years?

New mud. Every spring flood used to spill over the riverbanks and spread a fresh layer of sediment across the marshes, building them up as fast as they sank. The delta was like a sand castle being rebuilt by every wave.

Levees stop the spilling. That protects towns from floods, but it also funnels the river's sediment straight down the channel and off the edge of the continental shelf into deep water. The marshes still sink, but no new mud arrives to build them back up. Canals cut through the marshes and rising seas add to the loss.

Want proof? Look west along the coast, to the mouth of the Atchafalaya. That 30% of the flow the structure sends down the Atchafalaya carries sediment too, and there, at Wax Lake and the Atchafalaya's mouth, the coast is growing. Brand-new land is rising out of the water, visible from space. The delta isn't broken. The sediment is just going to the wrong places.

Holding the river has another cost, and it shows up in dry years. The Mississippi's bed near its mouth lies below sea level. When river flow drops, heavier salt water from the Gulf slides upstream along the bottom, underneath the fresh water, like a wedge.

In 2023, after a second year of drought, the wedge crept about 56 miles upriver. In Plaquemines Parish, about 2,000 people couldn't drink their tap water by mid-September. The Corps built an underwater barrier, a sill, across the riverbed to slow it, and warned that without action the salt could reach drinking water intakes as far north as New Orleans.

The takeaway: holding the river in place protects cities, but it starves the delta of the mud that keeps it above water. Louisiana is shrinking where the river is held and growing where the river is allowed to spread.

Hard question 3Can America hold the river forever?

Let's look at what's at stake first. The Mississippi is one of the busiest freight routes in the world. Barges carry grain, fertilizer, coal and fuel up and down it, and a single tow of 15 barges can carry about as much as a thousand semi-trucks. When the river ran low in 2022, barges had to carry lighter loads, and many sat stuck waiting for the water to rise. A river that suddenly moved would cut that route off from New Orleans and the ports below Baton Rouge.

That's why the system keeps growing. After the 1973 scare, the Corps added a second structure next door in 1986, the Old River Control Auxiliary Structure, to take pressure off the original. Upstream from Baton Rouge, the Morganza Spillway can be opened in giant floods to send extra water toward the Atchafalaya. It was opened in 1973 and again in 2011, when the river reached record heights.

Guess first: Every year the river doesn't jump, does the pressure to jump go up, go down, or stay the same?

It goes up. Remember the feedback loop: the Mississippi's own path keeps getting longer as it builds out at the mouth, while the Atchafalaya's stays short and steep. The difference in slope that drives the jump doesn't go away just because the gates are there. The river doesn't forget the shortcut.

So our prediction: the river will keep pushing as long as the shortcut exists. In ordinary years, the structures hold easily. The real tests are the giant floods, the 1973s and 2011s, when the most water is pushing hardest against them. Holding the river is not a single decision. It's a job that has to be done well, every flood, for as long as America wants the Mississippi where it is.

And scientists and engineers are working on the other half of the problem too: sending some river water and sediment back into the starving marshes on purpose, through controlled openings in the levees, to let the delta start building land again.

The takeaway: America can hold the river, but only by keeping up a constant, growing effort, and the river's pull toward the shortcut never stops.

More questions people ask

Try it yourself

Next time it rains, find a sandy slope or pour water slowly down a tray of damp sand tilted at one end. Watch the stream build a little fan of sand at the bottom. Keep pouring. Sooner or later, the stream will break sideways and start a new fan beside the old one. You've just watched a delta switch, the same thing the Mississippi has done for thousands of years.

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