Already, the Great Mississippi River Flood of 2011 has caused over $2 billion in damage, and the worst of the threat is still in front of us. Many are expecting that the total cost of flood damages—to crops, infrastructure, property destruction, and loss—to top $4 billion when all is said and done, which would make it anywhere from the fifth to the seventh most expensive flood since 1980 (PDF).
That $4 billion number is actually an optimistic estimate, considering the very real threat to the integrity of the Old River Control Structure, an absolutely massive and incredible infrastructure project built by the Army Corps of Engineers back in the 1950s that, basically, keeps the Mighty Mississippi from plunging into the Atchafalaya River bed,
Jeff Masters calls the Old River Control Structure “America’s Achilles’ heel” in a post that is absolutely a must-read for anyone who wants to better understand what’s happening in the Lower Mississippi at this very moment.
Basically, any river always wants to take the shortest and steepest route possible to its outlet. The Mississippi is no different, and since long before humans were around, it has jumped its banks every 1,000 years or so and charted a new, more desirable course. The image above shows the contrast between the long history of change in the Mississippi, and the Army Corp's plans for peak flow rates during massive 1-in-500 year floods. (Peter ran a nice full-length version of that first map in his post about the flooding Mississippi's impact on fish.) Sometime in the 1970s, the Mississippi River would’ve jumped its course and dumped into the much more inviting Atchafalaya River basin.
The mighty Mississippi River keeps on rollin’ along its final 300 miles to the Gulf of Mexico south of New Orleans—but unwillingly. There is a better way to the Gulf—150 miles shorter, and more than twice as steep. This path lies down the Atchafalaya River, which connects to the Mississippi at a point 45 miles north-northwest of Baton Rouge, 300 river miles from the Gulf of Mexico Delta. Each year, the path down the Atchafalaya grows more inviting.
The only thing preventing the river from taking that path is the Old River Control Structure. If that is compromised or breached, the impact would be enormous. The Mississippi would probably have a permanent new route to the Gulf of Mexico, and Baton Rouge and New Orleans—and all the towns in between—would be “stranded on a salt water estuary, with no fresh water to supply their people and industry.”
I really can't recommend highly enough that you read Masters' post in full. America's Achilles' heel is about to face its greatest test.