No one knows why the Hohokam Indians vanished. They had carved hundreds of miles of canals in the Sonoran Desert with stone tools and channeled the waters of the Salt and Gila Rivers to irrigate their crops for a thousand years until, in the middle of the 15th century, because of social conflict or climate change—drought, floods—their technology became obsolete, their civilization collapsed, and the Hohokam scattered. Four hundred years later, when white settlers reached the territory of southern Arizona, they found the ruins of abandoned canals, cleared them out with shovels, and built crude weirs of trees and rocks across the Salt River to push water back into the desert. Aware of a lost civilization in the Valley, they named the…