The Attacking Ocean by Brian Fagan Read Free Book Online Page B

world, since in one part of its journey it uses up one thousand three hundred miles to cover the same distance that the crow would fly in six hundred and seventy-five.” 1 This is a stupendous river by any standards, 3,779 kilometers long, with a huge triangular drainage area that covers about 40 percent of the United States, the third-largest river drainage in the world, exceeded only by the Amazon and the Congo. The river rises in Lake Itasca, Minnesota, and then flows through the heart of the Midwest. The Missouri River with its vast silt load, the Great Muddy, which drains the Great Plains, joins the Mississippi at St. Louis, the Ohio River at Cairo, Illinois. Below Cairo, the river flows through a wide, low valley, once a bay of the Gulf of Mexico, now filled with sediment. Today 966 kilometers downstream the Mississippi joins the Gulf. The river channel meanders over the low-lying plain, often contained between natural levees formed by flood sediments.
    Through a natural process known as avulsion, literally delta switching, the river has meandered back and forth across the flat landscape of the Lower Mississippi valley ever since sea levels climbed in the Gulf of Mexico after the Ice Age. The river’s gradient shallows with the ocean’s rise, the flow slows, and the silt load sinks to form lobes of a huge delta in a regime that changed little for thousands of years—until humans started controlling the Mississippi. Sediment builds up; a channel becomes clogged; the river shifts course to a steeper route downstream. Meanwhile the abandoned channel receives less water and becomes a bayou. A major channel shift triggered by an unusually severe spring flood takes place about every thousand years. The last one would have inundated maize fields on the floodplain, but the ancient farmers, hunters, and fisherfolk on the flatlands and among the bayous would have adapted to the shift without trouble. Today’s river is long overdue for a dramatic channel shift, most likely down the Atchafalaya Basin or through Lake Pontchartrain near New Orleans. This time the human and economic stakes are very high indeed, with almost unthinkable consequences for cities like New Orleans and Baton Rouge. Only massive humanly constructed flood control works stand between millions of people and disaster from upstream.

    Figure 13.1 Map showing locations in chapter 13 .
    Most of the Lower Mississippi’s water comes from the Ohio River and from downstream tributaries like the Arkansas and Red Rivers,only 15 percent from its own upper reaches. The Missouri contributes considerably less water, but massive quantities of silt, both of which flow down the Lower Mississippi. All these sources make for a complex flood regimen, especially when all major tributaries overflow at the same time. Such events resonate in historical memory. The Great Mississippi Flood of 1927 produced overflows so severe that the river reached a width of ninety-seven kilometers. On April 15, 385 millimeters of rain fell on New Orleans, covering parts of the city with more than two meters of water. The Flood Control Act of 1928 authorized the Army Corps of Engineers to construct the longest levee system in the world. With the new levees came at least some protection from raging floods, but also a new ever-present threat of highly destructive breaks in the defense walls caused by floods and hurricane-induced sea surges. For all the additional protection, many communities in the shadow of levees were potentially even more vulnerable than before.
    THE ROUTINE NEVER changed—enormous flocks of migrating waterfowl flying northward in spring, south in fall, along what is now known as the Mississippi flyway. Thousands of birds would pause to feed and rest at shallow oxbow lakes near the great river. Each spring and fall, the hunters would wait in the reeds at dawn with traps and spears. They would use canoes to drive the birds into narrow defiles in the reeds, where they could be