The book begins by describing how the Atchafalaya River drains 30 percent of the Mississippi River at its source 300 miles upriver from New Orleans. Thanks to its steeper gradient and more direct route, the Atchafalaya seeks to change the course of the Mississippi as has happened in its long geological history. Due to the Mississippis vital importance to industry, the Army Corps of Engineers constructed a control structure at the Atchafalayas source to prevent this from occurring and to maintain the 30 percent drainage. McPhee explains how Morgan City, Louisiana would be destroyed if the river's banks increase. Three million cubic feet of water would inundate the town in the case of a hundred-year flood, though the Corps of Engineers has been trying its hardest to build a more stable flood structure.