If you live in Louisiana and don't know how to swim, now 

 might be a good time to learn. 



— Shea Penland, "Taming the River to Let In 

 the Sea," Natural History, February 2005 



Less than seven months before hurricanes Katrina and Rita 

 brought indescribable devastation and heartache to the Gulf 

 coast, I warned the readers of this magazine, in the words 

 quoted above, of the inevitable path toward destruction that 

 New Orleans seemed determined to take. Of course, my near- 

 term prescience was unintentional, but my advice is still sound: 

 even if recent initiatives prove successful, you'll need at least 

 a pair of waders. Since the hurricanes hit, committees have 

 been formed, surveys have been commissioned, consciousness 

 has been raised. Most of the attention has gone into investigat- 

 ing how the levees failed and how best to rebuild them, even 

 though overambitious flood control was a prime cause oj the 

 conditions that made Katrina and Rita so deadly. 



Hundreds of years of natural-resource exploitation and 



modifications to the flow of the 

 Mississippi River — whose silty 

 waters created the delta region of 

 southern Louisiana — have cost 

 the state more than 1,900 square 

 miles of coastal wetlands hi the 

 twentieth century alone. The 

 U.S. Geological Survey estimates 

 that 213 square miles turned 

 into open water between the fall 

 of 2004 and the fall of 2005. 

 Natural History asked for my 

 reassessment of the issues I raised 

 from today's vantage point, two 

 years after the hurricanes. Because 

 so little had been done to address 

 the underlying problems, it seemed 

 apt to restate, in somewhat shorter 

 compass, what I said at that time. 

 My hope continues to be that 

 someone will listen. 



very year the threat 



of hurricanes looms. The 

 __ accompanying storm 

 surges cause local, short- 

 term flooding, but they 

 _ also erode away parts of 

 the coastal marshes and barrier 

 islands that provide the only 



Work continues on the floodgates of 

 the 17th Street Canal levee. A breach 

 in the levee flooded the Lakeview 

 area of New Orleans with more 

 than ten feet of water during the 

 aftermath of Hurricane Katrina. 



protection for the inhabited lowlands farther inland from 

 the Gulf. Louisianans have focused on river flooding for 

 hundreds of years, yet only in the mid-1970s did the state 

 begin to take seriously the problem of coastal erosion. 



Important as they are, though, beach erosion and flood- 

 ing are still not the heart of Louisiana's problem. That, in 

 a few words, is subsidence of the delta plain. 



Before the levees were built to channel and "control" 

 the Mississippi and other nearby rivers, floodwaters would 

 spread out and slow down as they flowed over the delta. 

 When the flow slowed, the river deposited its burden of 

 silt, forming a new layer of earth. But the levees, which 

 now constrict floods along a 1,200-mile corridor of the 

 Mississippi, prevent the floodwaters from spreading across 

 the delta. As a result, the river-borne silt is lost off the 

 edge of the continental shelf. 



The delta, primarily mud that had already filled the 

 Mississippi River valley before the levees were built, is 

 continuously being compacted under its own weight. As 

 it compacts, it loses elevation, and without floods, no new 

 sediments can arrive to build the land back up. In the past 

 several hundred years, subsidence rates have ranged from 

 one foot to four and a half feet per century. 



Compounding the risk of catastrophic flooding is global 

 climate change. Many climatologists expect such change 

 to cause hurricanes even more frequent and more violent 

 than those of the past several years. Sea levels are expected 

 to rise by ten to twenty inches. 



The threatened collapse of coastal Louisiana has been 

 centuries, even millennia, in the making. Eighteen 

 thousand years ago, toward the end of the last ice 

 age, sea levels began to rise dramatically. For thousands of 

 years the great glaciers that had formed in the preceding 

 era melted into the ocean. Eventually, four thousand years 

 ago, the sea level stabilized. But the Mississippi now met 

 the sea in what had been its old valley. The river water, 

 halted in its course by the Gulf of Mexico, no longer held 

 sufficient energy to carry its sediment. Falling out of the 

 flow, the sediment began to fill in the ancient river valley. 

 The result was a subsidence-prone delta that could main- 

 tain its elevation only as long as sediment from upstream 

 reached the delta plain each year. 



Meanwhile, over the millennia, the Mississippi Delta has 

 undergone a process known as delta-lobe switching. The 

 path the river takes to the Gulf of Mexico is continually 

 changing, because the river is continuously drawn along 

 the most efficient path to the Gulf. 



In the 1940s, in response to findings that the Mississippi 

 was about to switch course again, the U.S. Army Corps 

 of Engineers built a massive concrete structure known as 

 the Old River Control Structure, at the confluence of the 

 Red and Mississippi rivers, where the Atchafalaya begins. 

 This structure controls the volume of water that flows 

 down the Mississippi, allowing no more than 30 percent 



November 2007 naturai hisiorv 



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