The river “flows up the map,” they used to say, first south, then west, and then north, and through some of the most verdant and beautiful country in America. It is called the Tennessee, but it drains some forty thousand square miles of land in seven states, from the Blue Ridge Mountains to Alabama, and from Mississippi to the Ohio River, an area nearly the size of En­gland.

Before the 1930s, it ran wild, threatening each spring to flood and wash away the humble farms and homes along its banks. Most of it was not navigable for any distance, thanks to “an obstructive fist thrust up by God or Devil”—as the writer George Fort Milton characterized it—that created a long, untamed run of rapids known as Muscle Shoals. The fist dropped the river 140 feet over the course of 30 miles, and therein lay the untapped potential of the Tennessee, the chance to make power—a lot of it—out of water.