the Morganza spillway dam and run down the Atchafalaya (pronounced 'Chafalaya) as it almost did in 1972-73. During that flood, the dam was undermined and the entire structure was vibrating and rumbling, something that that gave the Corps of Engineers their worst frght of the century. They almost lost the Mississippi River down the Atchafala River channel, and New Orleans would have been isolated, left high and dry with the new river channel way over by Texas.
First some hydrogeology. The Mississippi River meanders from side to side during periods of flooding on its vast floodplain. The channel is ever changing, and has been doing so for the past 5,000 years. The upstream portion of the river has a V-shaped channel that is typical of most streams on solid bedrock, with steep gradients and low dischage rates. But once it hits Cairo, Illinois, it spreads out over the Gulf Coastal Plain, which is made up of Cretaceous and Tertiary sands and gravels. Once here, the gradient flattens out and the sediment load increases.
Without a steep gradient, the river energy cannot be used to incise the channel deeper, and the energy is dissipated with a sinuous side-to-side motion, like a sidewinder rattlesnake. During floods, the channel overflows, and dumps sediment where it breaks over the bank, forming a natural levee. Over time, these levees can get tens of feet high, restricting the channel.
Provide a larger flood, and the channel can break through the natural levee and incise a newer channel, leaving behind a series of ox-bow lakes, that you can see on the floodplain all along the lower reaches of the big river.
Now let's look downstream. When the Mississippi gets to the Gulf of Mexico, the sediment load drops out completely and forms the delta as the sands and muds spread out in the still Gulf waters. It will send a jet of muddy water far out into the Gulf that forms a finger-like projection.
Back upstream, a levee break that forces the river into a new channel may send a distributary finger into another Gulf location entirely, sometimes hundreds of miles to the east or west. Do this often enough over thousands of years and you get a classical birdfoot delta.
Now back to the Morganza spillway and the actions of the US Army Corps of Engineers over the past 150 years. The Corps knew that NOLA could be bypassed if the river jumped its channel upstream, so all of their efforts have gone into strengthening levees and keeping the river confined in its present channel. You can see how far out this distributary finger extends into the Gulf, compared to prior distributaries.
But one of these years, the channel jump down the Atchafalaya is going to happen, and no forces on earth can repair the breach. Take a look at the New Yorker article by John McPhee(great reading).
Some quotes: In time, people would come to suggest that there was about these enterprises an element of hauteur. A professor of law at Tulane University, for example, would assign it third place in the annals of arrogance. His name was Oliver Houck. “The greatest arrogance was the stealing of the sun,” he said. “The second-greatest arrogance is running rivers backward. The third-greatest arrogance is trying to hold the Mississippi in place. The ancient channels of the river go almost to Texas. Human beings have tried to restrict the river to one course—that’s where the arrogance began.”
“The more water the Atchafalaya takes, the bigger it gets; the bigger it gets, the more water it takes. The only thing that interrupts it is Old River Control. If we had not interrupted it, the main river would now be the Atchafalaya, below this point. If you left it to its own devices, the end result had to be that it would become the master stream. If that were to happen, below Old River the Mississippi reach would be unstable. Salt would fill it in. The Corps could not cope with it. Old River to Baton Rouge would fill in. River traffic from the north would stop. Everything would go to pot in the delta. We couldn’t cope. It would be plugged.”
Some things can't be controlled, ever.