USA: The Mississippi river is under control—for now
By Boyce Upholt
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The Missouri, the Ohio; the Red, the Illinois, the Arkansas; the Pecatonica, the Poteau, the Big Sioux—across the U.S., rivers have swollen this year, swamping homes and cropland, costing farmers billions of dollars. Running through more than a million square miles of the heart of the United States—40% of its land area—100,000 waterways eventually drain into the Mississippi. Over 30 million people live near the Mississippi or one of its tributaries. There, on the big river, the Army Corps spent the last nine months trying to contain its longest recorded flood, the latest in an increasingly devastating series.
Scientists blame much of the epidemic of flooding on the Corps’ own engineering. In order to protect farms and cities, they built humps of earth along the Mississippi’s banks—levees, which narrowed the river, forcing it to rise. Meanwhile, countless acres of urban pavement have replaced absorbent soil. Pair these changes with the wettest 12 months in recorded history, and the Mississippi River becomes a menace.
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On the lower Mississippi River—which begins in Cairo, the southernmost city in Illinois—flood control is far more advanced than in the Midwest. In 1927, floods demolished 26,000 sq. mi. of the South, killing as many as 500 people. It was the era’s Katrina, a disaster that gripped the nation. The federal government decided that the region should never be wet again, and the Army Corps was tasked with the design and construction of the Mississippi River & Tributaries Project to seize control. The [Mississippi River & Tributaries Project] is so vast that it can be hard to comprehend as a single object: it includes concrete floodwalls in New Orleans and Cairo and Caruthersville, Mo.; pumping stations that drain rainwater trapped behind these walls; and nearly 3,500 miles of levees along the river and its tributaries. A fleet of towboats and barges, outfitted with cranes and sleeping quarters for a crew of 200, descends the river each year, paving its bends with concrete to halt erosion. In the 1930s and ’40s, dredge boats straightened the river, shortening it by 150 miles, so that floodwater would speed more quickly downstream.
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Built in pieces over decades and still incomplete, the MR&T is designed to hold the “project design flood.” This hypothetical disaster, dreamed up in 1954, represents the largest probable flood based on historical data available at the time. The levees are designed to handle only a portion of this flood; when a trigger point is met, the Corps is authorized to open one of the MR&T’s four floodways, sending the excess water down a different route to the Gulf of Mexico. As of 2010, the Corps expected this would be necessary about once every 10 years. Bonnet Carré has been opened three times in the past 18 months.
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There is no way to predict when the Big [Flood] will come, or what element of the system might fail. But this year’s high water signals that the Corps’ “project design flood” is already distressingly out of date. The engineering was designed around discharge readings in 1927, but today, perhaps because the flood-prevention structures have narrowed the river, the same discharge often leads to higher water than it did 90 years ago. In May, the Corps was forced to open Bonnet Carré before the discharge trigger was met because the water had risen so high.
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