It was the Great Mississippi Flood of 1927 – the worst river flooding in American history – that prodded Congress to direct the US Army Corps of Engineers to begin an intensive levee-building program in Louisiana.
Those levees and their successors accomplished Congress's goals: They kept the Mississippi River open to shipping and out of New Orleans. But they also represented a Faustian bargain that Louisiana state engineers recognized at least as early as the 1840s. By keeping silt in the channel rather than letting it build up banks or replenish the surrounding landscape, levees raised the river and allowed the land to sink. The region is now trying to deal with the ecological and geophysical consequences.
Even 160 years ago, engineers understood the dangers of the river's rising water levels.
"Every day, levees are extended higher and higher up the river – natural outlets are closed – and every day the danger to the city of New Orleans and all the lower country is increased," the state engineer, P.O. Herbert, observed in 1846. Louisiana should "endeavor to reduce this level, already too high and too dangerous, by opening all the outlets of the river," he wrote in a report.
Four years later, his successor, A.D. Wooldridge wrote: "I find myself forced to the conclusion that entire dependence on the leveeing system is not only unsafe for us, but I think will be destructive to those who shall come after us."
Levees also were destructive in ways Herbert and Wooldridge didn't imagine. Muck that once replenished the countryside when the river flooded now rolls out the river's mouth and slides down the continental shelf. All that sediment accumulated over thousands of years weighs heavily on the delta region's crust, causing it to sink. And that sinking – or subsidence – is happening faster than the buildup of new silt.
The subsidence problem can be worse in localized areas where, for example, levee and canal systems seal off sections of land from periodic flooding – as in New Orleans. The result: the organic-rich soil dries out and the organic material decomposes. What's left takes up a small fraction of its former volume, causing the land to sink.