The Colorado River’s 1,450-mile run begins amid the snowy pinnacles of the Rocky Mountains and ends in the subtropical waters of the Gulf of California. Over the millions of years the river has been running this course, it has gradually carved through the Southwest’s crimson limestone and shale to create a succession of unimaginably vast canyons: Ruby, Cataract, Marble, and Grand. The writer Marc Reisner described the Colorado as the “American Nile.” The Hualapai call it Hakataya, “the backbone.”