The Control of Nature

The title of this post is taken from John McPhee's 1989 book of the same name. For anyone who wants to better understand the U.S. Army Corps of Engineers and its history on the Lower Mississippi, the book--or, rather, the first part of the book, called "Atchafalaya"--is required reading. In that chapter, McPhee trains his sights on the Old River Control Structure, which has kept the Atchafalaya River from "stealing" the Mississippi's flow and shunting the delta more than 100 miles west. As today's Earth Observatory entry explains:

The delta switching has occurred every 1,000 years or so in the past. As sediment accumulates in the main channel, the elevation increases, and the channel becomes more shallow and meandering. Eventually the river finds a shorter, steeper descent to the Gulf.

Namely, the Atchafalaya. Such a shift would have left New Orleans and Baton Rouge effectively high and dry, something Louisiana and Congress obviously couldn't countenance, and so the Old River Control was built. The repercussions of bending the river to our needs are of course still being felt. As Michael Grunwald notes in the article referenced in the previous post:

The Corps has been at war with the Mississippi for a century, and the massive levees it built along the river have helped keep middle America dry. But this war on nature has had unintended consequences, choking off the river's natural land-building process. The straitjacketed Mississippi no longer carried as much silt from its banks and its floodplain down to its delta, so it no longer created as many of the coastal wetlands that served as natural hurricane protection for New Orleans. The city was now safe from the river, but dangerously exposed to the gulf; 25 square miles of protective wetlands vanished every year -- partly because of the oil industry, but mostly because of the Corps. And since the huge silt infusions that had shored up the city's foundations no longer arrived, New Orleans began to sink.

Overall, scientists believe these land losses raised Katrina's surge by several feet.