Crossposting my MOTF entry:
A very recent POD this time: the major Mississippi River floods of 2011 are a little more major and a little less well-handled. The Old River Control Structure and Morganza Spillway, which keep the river on its current course through Baton Rouge and New Orleans, fail, and the main flow of the river switches into the Atchafalaya, taking a steeper, shorter route to the Gulf. The economic consequences, especially in the wake of Hurricane Katrina, are terrible. The ports of New Orleans and Baton Rouge become unusable as the river narrows and loses depth, putting strain on oil and food supplies nationwide. The country recovers, but New Orleans will never be the same again. Tens of thousands of jobs disappear. Many who left after Katrina never return. The port facilities are not upgraded upon the Panama Canal's 2016 expansion, and are soon obsolete: the new size of Panamax couldn't come to dock even if the water were deep enough. Tax revenue dwindles, crime rises, and the streets empty out. Tourism, which so many had counted on to sustain the city in the absence of trade, begins to falter as the urban landscape decays. The city becomes a husk, half depopulated, soon more than half. Over the decades, Gulf waters will rise up and the former river delta subside. Parts of the French Quarter will be disassembled and rebuilt in Las Vegas, but the rest of what was New Orleans will one day slip, mourned but not defended, beneath the waves.
This map is a from a ten-year retrospective article on the decline of New Orleans, with Baton Rouge and Morgan City (near the mouth of the Atchafalaya) shown as well. Baton Rouge has suffered badly but has retained its massive oil refineries (now with more complicated logistics) and status as capital, and so will survive. Morgan City was virtually wiped out in the initial flood of the Atchafalaya. It has been rebuilt, though the new city and old town have little in common beyond the name--the original population has been swamped by newcomers constructing and operating its ultra-modern new port. In fact, the whole Atchafalaya basin is filling with people. Once the heart of Cajun Country and home to some of the better preserved natural lands in the state, the region's distinctive character is already being swept aside by the torrent of wealth and activity that is the Mississippi River.
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This idea started with a much earlier POD, with New Orleans as a city state and the river intentionally diverted by its unfriendly northern neighbors, but I couldn't quite get it to hang together. As I researched the Atchafalaya capture scenario, I realized that there was a perfectly good POD in recent memory, with a bit more story already baked in: Katrina weakening and temporarily depopulating the city five years earlier, the Panama Canal expansion potentially leaving a failing NOLA behind five years later. I don't really have any idea whether my water level for the Mississippi is reasonable--I can't tell from what I've read how quickly the switch would take place, only that a threshhold would be crossed and the main current could not be recovered. I have a feeling that the map shows too wide a river--maybe the state has deliberately diverted what it could that direction?