Six hundred miles long from its source in the mountains of southwestern New Mexico to its confluence with the Colorado River above Yuma, the Gila has been an important avenue for the movement of birds, animals, plants, and peoples across the desert for millennia. Many cultures have sprung up on its banks, and millions of people depend on the river today—whether they know it or not. Gregory McNamee, author of the prizewinning book Gila: The Life and Death of an…