Left to themselves, rivers flowing through flat country change course all the time. There’s a positive feedback mechanism: If a loop or bend starts, the water has to flow faster to get to the end in the same time, and “centrifugal force” makes it faster on the outside of the loop. That erodes the outside bank and deposits sediment on the inside, and the loop grows bigger and bigger until the same effect at the “neck” of the loop wears through, whereupon the loop becomes an isolated slough and the river flows straight until the next loop starts to form.
Most of the middle part of the United States is relatively flat, largely because the Mississippi and its tributaries have eroded it down over the eons. The natural course of those rivers would be a series of growing and shrinking loops, with annual floods spreading over wide expanses of surrounding land. That’s not convenient for the people living nearby. The loops increase the length, making barge traffic take longer, and an incredible amount of American commerce goes by barge up and down the Mississippi, the Ohio, and the rest of the system. Some of the most valuable farm land in America is found in the flood plains, where the river has deposited sediment in earlier epochs, and if the river changes course that farmland can disappear under water. That isn’t the worst part, because a roughly equal amount of new land would then be exposed — but our land use patterns are based on fixed boundaries, and if the river takes a hundred acres away from one farmer and gives it to another, it causes all kinds of legal problems. Rivers are also commonly State or National boundaries, and that can be a big legal problem. There are already many places where the boundary isn’t the river any more, it’s where the river was at some earlier time.
That’s especially important at a place called Old River. In a delta, which is by definition quite flat, rivers deposit sediment that eventually blocks their path, which results in the water seeking a new outlet. Every river delta has multiple mouths that switch off over time as the sediment builds. The channel that passes through Baton Rouge and New Orleans has been building up sediment for the entire time the country has been settled, and water had begun to flow via the Old River through the Atchafalaya, bypassing the settled areas. That process would naturally result in the Mississippi using a new mouth of its delta, ‘way to the west of the main settlements, and that would be a financial problem verging on disaster, not just for the people of Baton Rouge and New Orleans but for the country as a whole. There are billions, possibly trillions, of dollars in capital infrastructure along the existing path — barge and ship terminals for everything from exported grain to imported LNG, and the support for those — that would be rendered worthless by the change in the river’s course. At best they would all have to be rebuilt. At worst it might mean moving two major cities lock, stock, and liquor stores. Keynesians would rejoice, but that wouldn’t just be a broken window, it’d be tearing down the shop and the city it’s in.
So the Corps of Engineers built the Old River Control Structure, which forces the river to continue on its present path, and the Morganza Spillway, which is designed to relieve pressure on the Old River structure when conditions make it necessary. That has had all kinds of undesirable side effects. The major one, from the point of view of the engineers, is that the buildup of sediment has continued, requiring building higher and higher levees along the river banks. There are lots and lots of places where the river is considerably higher than the surrounding land, and if the levees ever break it’ll be like popping a balloon — it’s hard to tell just where the water would go, but it’s certain that wherever it went it’d be expensive for lots of people. Another is environmental problems. In order to keep the river moving at all, and therefore transporting at least some of the sediment to the Gulf of Mexico instead of blocking traffic, more and more water has to be forced into the main channel, and that starves the rest of the delta of sediment. Looking at a time-lapse map of Louisiana can be eye-opening. Places that used to be dry land are now swamp; places that used to be swamp are now open channels, many of them to the Gulf, because there’s no sediment to keep them built up. Not only is the land area of Louisiana shrinking at a remarkable rate, the simplification of the delta means if the water does start flowing it will move fast instead of slowing and spreading. Levees and spillways and control structures have turned into a Red Queen’s race, with the engineers running as hard as they can to keep the present conditions in place.
Now we have major flooding in the entire Mississippi river system, and face Hobson’s Choice: either let some of that water go where Nature would have put it long ago, or let several major cities along the rivers flood out completely. It’s already been done further upstream, to the vocal displeasure of local land and business owners and Governments, but if the Corps of Engineers loses the race and the Mississippi goes to Morgantown the diversions in Illinois and northern Missouri will look like kids playing mud-pie.
Letting Baton Rouge and New Orleans turn into backwater (literally!) villages would be a big economic hit, but that would play out over some time. The immediate problem would be the oil industry. A vast amount of oil passes through South Louisiana because the network of delta channels allow easier access for tankers than is available in Houston, Orange/Port Arthur, or Mobile. There are refineries, storage tanks, pipeline facilities, tanker offloading ports, and a thousand thousand other bits scattered over the whole area that might get flooded, and parts of the Strategic Petroleum Reserve would be affected. Even shutting them down or curtailing operations to cope with the unexpected water will form a bottleneck that will make it harder to get fuel to the people who need it, and raise the prices. If they get destroyed the impact will be huge.
Greens will approve, because the new path would restore sediment deposition to an area that’s been starved of it for years with huge impact on the ecosystem and habitat. I probably ought to cheer, because that would be very good for Texas, which is the only place even partly prepared to take up the slack. Overall, though, it would be a bad thing for everybody in the country, as the oil business, already suffering from natural, technical, and Governmental disasters, reels from the impact. Keep your eye on the news. Morganza is intended to relieve pressure on Old River, and the impact of opening Morganza is going to be big. If they ever talk about opening Old River, be prepared for the job of essentially rebuilding everything economically important in a triangle whose corners are approximately Lake Charles, Freeport MS, and Memphis. The idiom used in my childhood was “Katie, bar the door”. In this case, though, Katie barred the door long ago, and is now trying to cope with battering rams.
 Yes, an oversimplification. It’s useful as a first approximation.