In the 1840s and ’50s, most pioneers reckoned that the nation’s great opportunities lay beyond this parched region, at the end of the trails, in Oregon and California. But then the idea of the Great American Desert was turned on its head.

“These great Plains are not deserts,” wrote William Gilpin in a late-1857 edition of the National Intelligencer, “but the opposite, and are the cardinal basis of the future empire of commerce and industry now erecting itself upon the North American Continent.” As the soon-to-be first territorial governor of Colorado, the electric-tongued Gilpin knew that he would benefit from this novel outlook. He shamelessly rode the tide of Manifest Destiny—the notion that Americans were destined to push across the continent—to encourage westward migration.

His strategy worked. As soldiers came home from the Civil War and the nation returned to the business of business, all America, it seemed, scrambled aboard Gilpin’s bandwagon. “Go West, young man!” soon rang in everyone’s ears.

The railroads—America’s most visible instrument of its Manifest Destiny—adopted such visions enthusiastically. By far the largest private landowners in the West, they pushed out tides of promotional material that sold the West as a garden, not a desert. Utah was a promised land, proclaimed the Rio Grande and Western Railroad. “You can lay track through the Garden of Eden,” said Great Northern Railroad’s founder, James J. Hill, “but why bother if the only inhabitants are Adam and Eve?” The brochure in the pocket of the man who froze to death may well have come from a railroad’s well-oiled marketing machine.

Rolling in to support these cheerleaders came “science.” The “rain follows the plow” theorists—among them, many prominent scientists—became the chaplains of the western movement. By simply cultivating the arid soil, they postulated, the local climate would change permanently; the number of rainy days would rise and crops would burst out of the fertile ground.

Other even more outlandish ideas took root: The smoke from trains supposedly caused more rain, as did planting trees. The secretary of agriculture even traveled to Texas to evaluate the popular idea that dynamiting the atmosphere would induce rainfall. These were all flat-out fantasies, but, combined with a period of unusually good weather, they were highly enticing.

The populations of Nebraska, Kansas, and Colorado doubled or tripled in the years following the Civil War.

In April 1877, when John Wesley Powell stood in front of the National Academy of Sciences’ annual meeting in Washington, D.C., everyone in the room knew the details of his daring passage on a tiny rowboat down the Colorado River and through the Grand Canyon eight years before. No mere tough guy, Powell was a formidable surveyor who would soon head the U.S. Geological Survey. He had climbed, walked, and boated through more of the American West than any white man, and had something to say about William Gilpin’s astounding claims.

He unrolled a document carefully with his left hand—he’d lost his right arm during the Civil War—to reveal a map of the continental United States. On it, he had drawn a vertical line, technically an isohyet, beginning in central Texas and rising up through Kansas, east of Nebraska, and through Minnesota and the Dakotas, approximating the 100th meridian. This startlingly simple line forced its viewers to visualize the American nation not in terms of political boundaries, but by its climate: It delineated the arid West from the forested East, land that received 20 or more inches of rain from that which received less. He chose 20 inches as his dividing point because that was the minimum necessary to conduct conventional agriculture without irrigation. The map illustrated forcefully how much of the American West, with some notable exceptions in the Pacific Northwest, was unfarmable.