The river has its source on the western slopes of the continental divide in Yellowstone National Park, flows south through Grand Teton National Park, curves west in a long arc through southern Idaho, then turns north and west for its meeting with the Columbia River, 1,038 miles from its beginnings. The land along its southern arc is called the Snake River Plains, and at the southernmost point of the arc there is a place called the Magic Valley—unsurprisingly, for a kind of magic was done there more than seventy-five years ago.

Magic was needed. The Snake River Plains—arid, treeless, clotted with sagebrush—once was one of the most desolate spots on earth, a stretch that overlanders on their way to California and Oregon in the 1840’s and 1850’s crossed just as fast as tired feet and lame oxen would take them. “It is a land where no man permanently resides,” Washington Irving wrote in 1837, “a vast, uninhabited solitude, with precipitous cliffs and yawning ravines, looking like the ruins of a world; vast desert tracts that must ever defy cultivation and interpose dreary and thirsty wilds between the habitations of man.” And for most of the nineteenth century that was how the Snake River Plains and the Magic Valley would remain, waiting, like most of the arid West, for the outcome of one of the most far-reaching movements in American history.

The movement lay close to the heart of what Americans thought of themselves. “Cultivators of the earth,” Thomas Jefferson once wrote, “are the most valuable citizens. They are the most vigorous, the most independent, the most virtuous, and they are tied to their country, and wedded to its liberty and interests, by the most lasting bonds.” But by the last decades of the nineteenth century, the stunning growth of American industry had largely unraveled this fine vision of the yeoman farmer as the prototypical American; work now was in the cities; arable land in the East became scarce for a burgeoning population; American society seemed destined to degenerate into a division between working-class have-nots and upper-class haves, with nothing in between.

For many of those appalled by such a prospect, the answer appeared to lie in the lands west of the hundredth meridian, a country rich in rivers and soil but with an average annual rainfall of less than twenty inches. The solution, they reasoned, was simple: irrigation. Beginning in 1891 a series of annual and biannual irrigation congresses were held whose members issued the frequent cry of “a million forty-acre farms” as their goal, even though John Wesley Powell— whose 1878 Report on the Lands of the Arid Regions of the United States was, and still is, the most trenchant work on the subject—stood up before them in 1893 and warned, “I wish to make it clear to you, there is not enough water to irrigate all these lands; there is not sufficient water to irrigate all the lands which