From Blood and Thunder: The Epic Story of Kit Carson and the Conquest of the American West, by Hampton Sides (Anchor, 2007), Kindle Loc. 4435-4454:
North America had never seen such a florescence of culture. But then around A.D. 1150, just as quickly as they had burst upon the scene, the Chacoan culture ebbed. The agent of their demise seems to have been an environmental collapse brought on, in part, by two devastating droughts in 1085 and 1095, and in part by the impact of a dense population living on a marginal desert landscape. Their expansion had been predicated on a kind of meteorological accident; they had been living in a hundred-year cycle of aberrant wetness, and during that brief window the Anasazi in Chaco Canyon had overfarmed, overhunted, and overlogged. In only a few generations their deforested land became eroded, the topsoil depleted, the drainages choked with salt and silt. The river on which they depended for corn and beans dried up. A third major drought, beginning around 1129, delivered the final blow.
This environmental upheaval led, predictably enough, to a social upheaval. In its death agonies, Chaco Canyon was not a pleasant place. People began to starve. They turned their great houses into fortresses, erecting walls, barricading the first-floor windows, retreating to higher stories at night, pulling up their ladders at the slightest hint of danger. Archaeologists have found evidence of widespread civil unrest, witchcraft, and even ritual cannibalism. Finally, the Chaco Anasazi began to leave in large numbers. In many cases they simply walked away from their great apartment complexes, leaving behind beautiful ornamental pottery, sandals and clothing, and large quantities of dried food neatly stored in granaries.
But the Chacoans did not really “vanish.” They wandered all over the Southwest, resettling wherever they could find water and safety. The Pueblo Indians were their direct descendants. Hosta and his Jemez people had Chaco blood, not Aztec blood, coursing in their veins, and so did the dozens of other Pueblo tribes whose settlements formed a loose constellation across the New Mexico Territory—the Zuni, the Hopi, the Acoma, the Taos Indians. The architecture of the modern Pueblos, though not as technically sophisticated, was strikingly similar to that of the Chaco great houses, with their multistoried apartments, retreating terraces, and underground kivas. From Puebloan rock art to their religious ceremonies to the styles of their pottery, the cultural echoes were clear. The Chaco Anasazi remained alive and well; they had simply undergone a diaspora of sorts. In spreading out and regerminating in smaller hunkered settlements, the descendants of the Anasazi learned the final cautionary lesson of Chaco Canyon: the peril of density in the face of the desert. In a meager landscape, civilization must scatter.