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What complicates the story of the destruction--and makes the loss of life so tragic--is our role in the disaster. If it's true that human activity had a lot to do with making the region vulnerable to a hit by a hurricane like Katrina, it's also true that we knew all along the kind of environmental damage we have been doing to the Gulf. Now that we're paying the price for our recklessness, what are we going to do about it?
It was always evident that the Gulf of Mexico was a sweet spot for cyclones, but it took modern meteorology to explain just why. You need a lot of things to get a hurricane going, most important among them an existing storm with a bit of spin to it wedged between warm ocean water and a colder band of air above it. Locate all that at least 300 miles north or south of the equator--where the rotation of the Earth's slightly narrower circumference exacerbates the spin of the storm--and you have everything you need to sustain a hurricane. The Gulf has all those ingredients, and its cities and towns repeatedly suffer for it.
To survive such storms, early residents quickly learned that they would have to build carefully, particularly in low-lying New Orleans. Eighteenth century settlers established the famed French Quarter on some of the highest ground they could find, one of the reasons it remained relatively dry last week. As the Gulf, the lake and the river periodically overflowed, the growing city retreated behind an ever expanding web of soil, concrete and metal levees. Today there are 350 miles of those barricades snaking through the city and 22 massive pumping stations that are supposed to kick into action whenever the water sloshes over the walls. Having constructed that elaborate system, New Orleans was not inclined to abandon it. "The city built the levees to protect itself," says Craig Colten, L.S.U. geographer and author of the book An Unnatural Metropolis. "Now there's a huge investment in drainage."
Geology has only made things worse. Gulf land is squishy stuff, made mostly of silt deposited by eons of free-flowing rivers and periodic floods. When the high water recedes, the sedimentary layer remains, growing heavier and heavier and ultimately subsiding under its own weight. The only way to keep the land from sinking altogether is to let the soil replenish itself with each flood. Human beings have done just the opposite, walling off New Orleans and re-engineering the Mississippi River to flow around the growing metropolis, effectively choking off the silt supply.
In addition to allowing the unreplenished coastal marshlands to sink, that tampering eventually kills the wetlands that do survive, as salt water intrudes deeper and deeper inland, killing vegetation that helps hold the soil together. The elimination of natural flooding also causes barrier islands, which line the Gulf and protect the coast, to shrink. The Mississippi in its naturally flowing state spilled silt into an intricate delta, spreading sediment east and west and fortifying the islands. Walled and dredged all the way to the Gulf, the river now dumps that silt right over the edge of the continental shelf. Geologists report that the Chandeleur Islands--a healthy necklace of sandy barriers about 70 miles from New Orleans--appeared to have been wiped out by Katrina, leaving one more stretch of the city's coast dangerously exposed.