provided funds for coordinated river defenses that were still incomplete in the 1980s. This time, the corps did more than raise levees. They straightened parts of the river, built spillways, like that at Bonnet Carré, nineteen kilometers west of New Orleans, which diverted water away from the city and into Lake Pontchartrain during a major flood in 1937. In places, levees now rose to more than nine meters, but increasingly the Army Corps turned its attention to the Atchafalaya, which flowed into one of the largest river swamps in North America.
Without human interference, the Lower Mississippi’s floodwaters would disperse widely over the delta plain through many outlets. Virtuallythe entire delta would be covered not only with floodwater but also with fine sediment derived from mountains hundreds of kilometers away. As the writer John McPhee remarks, “Southern Louisiana is a very large lump of mountain butter, eight miles thick where it rests upon the continental shelf, half of that under New Orleans, a mile and a third at Red River.” 5 Deposits like this compact, condense, and sink. McPhee calls the delta “a superhimalaya upside down.” The subsidence continues despite human intervention. Until about 1900, the Mississippi and its tributaries compensated for the subsidence with fresh sediment that came down each year. The delta accumulated unevenly but on the positive side of the geological cash register, as channels shifted and decaying vegetation sank into the flooded silts. The vegetation itself grew as a result of nutrients supplied by the Mississippi.
Before the days of flood defenses, the river spread across the surrounding country quite freely and in many places, except at low water, when it stayed within its natural banks. Today, over three thousand kilometers of levees constrain the river until Baptiste Collette Bayou, ninety-seven kilometers downstream of New Orleans. The delta has lost silt for a century and southern Louisiana is sinking. Meanwhile the river with its levees shoots fine river sediment out into the Gulf of Mexico—some 356,000 tons of it a day. The water rises ever higher behind the levees as the surrounding landscape continues to subside. McPhee calls the delta “an exaggerated Venice, two hundred miles wide—its rivers, its bayous, its artificial canals a trelliswork of water among subsiding lands.” 6
The entire delta is a highly vulnerable, threatened landscape. About half of New Orleans is as much as 4.6 meters below sea level, hedged in between Lake Pontchartrain and the Mississippi. The richest inhabitants live on the highest ground by the river. The poorest dwell at lower elevations in the most vulnerable locations of all. The city receives abundant rainfall, often torrential downpours that cause serious flooding within the river defenses, as happened with Tropical Storm Isaac in 2012. There are no natural outlets, so the water has to be pumped out, which tends to lower the water table and increase subsidence. The pumps, invented by engineer A. Baldwin Wood during the early twentieth century, allowedthe city to expand to ever-lower elevations; there was no space elsewhere. 7 New Orleans is so waterlogged that the dead are buried in cemeteries aboveground. Even from the slightly higher ground of the French Quarter, you look up at the levees and passing ships, their keels well above the city streets.
New Orleans has distinctive levee problems, not least that of the waves caused by the wakes of passing vessels. Everywhere, the high water line is rising as the speed of the confined water from upstream increases and the levees themselves subside. Down at the coast, the shortage of river silt has led to erosion, to the tune of some 130 square kilometers of marshland a year. Louisiana is 4 million square kilometers smaller than it was a century ago. Half a kilometer of marsh reduces the height of a storm surge from offshore by 2.5 centimeters. With the disappearance of 130 square
Without human interference, the Lower Mississippi’s floodwaters would disperse widely over the delta plain through many outlets. Virtuallythe entire delta would be covered not only with floodwater but also with fine sediment derived from mountains hundreds of kilometers away. As the writer John McPhee remarks, “Southern Louisiana is a very large lump of mountain butter, eight miles thick where it rests upon the continental shelf, half of that under New Orleans, a mile and a third at Red River.” 5 Deposits like this compact, condense, and sink. McPhee calls the delta “a superhimalaya upside down.” The subsidence continues despite human intervention. Until about 1900, the Mississippi and its tributaries compensated for the subsidence with fresh sediment that came down each year. The delta accumulated unevenly but on the positive side of the geological cash register, as channels shifted and decaying vegetation sank into the flooded silts. The vegetation itself grew as a result of nutrients supplied by the Mississippi.
Before the days of flood defenses, the river spread across the surrounding country quite freely and in many places, except at low water, when it stayed within its natural banks. Today, over three thousand kilometers of levees constrain the river until Baptiste Collette Bayou, ninety-seven kilometers downstream of New Orleans. The delta has lost silt for a century and southern Louisiana is sinking. Meanwhile the river with its levees shoots fine river sediment out into the Gulf of Mexico—some 356,000 tons of it a day. The water rises ever higher behind the levees as the surrounding landscape continues to subside. McPhee calls the delta “an exaggerated Venice, two hundred miles wide—its rivers, its bayous, its artificial canals a trelliswork of water among subsiding lands.” 6
The entire delta is a highly vulnerable, threatened landscape. About half of New Orleans is as much as 4.6 meters below sea level, hedged in between Lake Pontchartrain and the Mississippi. The richest inhabitants live on the highest ground by the river. The poorest dwell at lower elevations in the most vulnerable locations of all. The city receives abundant rainfall, often torrential downpours that cause serious flooding within the river defenses, as happened with Tropical Storm Isaac in 2012. There are no natural outlets, so the water has to be pumped out, which tends to lower the water table and increase subsidence. The pumps, invented by engineer A. Baldwin Wood during the early twentieth century, allowedthe city to expand to ever-lower elevations; there was no space elsewhere. 7 New Orleans is so waterlogged that the dead are buried in cemeteries aboveground. Even from the slightly higher ground of the French Quarter, you look up at the levees and passing ships, their keels well above the city streets.
New Orleans has distinctive levee problems, not least that of the waves caused by the wakes of passing vessels. Everywhere, the high water line is rising as the speed of the confined water from upstream increases and the levees themselves subside. Down at the coast, the shortage of river silt has led to erosion, to the tune of some 130 square kilometers of marshland a year. Louisiana is 4 million square kilometers smaller than it was a century ago. Half a kilometer of marsh reduces the height of a storm surge from offshore by 2.5 centimeters. With the disappearance of 130 square
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