Drought and the end of the Chaco world

Chaco Canyon in north-western New Mexico had been the centre of the San Juan Basin since about 1050: some fifteen great houses of planned masonry, linked by straight roads to outlying communities — at least seventy of them by 1115 — served as places of ceremony and, some suggest, of trade. Building went on into the early 12th century — Wijiji, by one account the last Chacoan great house, was built in 1110–1115, and the great houses at Aztec and Salmon on the Animas and San Juan rivers rose during a wet period that began around 1100 — but the construction of great houses in the canyon had ceased by about 1130. Around 1130 an extreme drought set in that lasted about half a century, its most severe phase falling in 1140–1162, and the cohesive Chacoan system began to unravel around 1140.

It has been suggested that even in wet periods the canyon could sustain only about 2,000 people. Poor water management led to arroyo cutting, deforestation was extensive and economically devastating, and timber for building already had to be hauled from the Chuska Mountains, more than 80 km to the west. Outlying communities were depopulated; during the drought most great houses of the central San Juan Basin were vacated, and by one account the buildings of the central canyon had been neatly sealed and abandoned by the end of the century. Some scholars suggest that violence, perhaps involving cannibalism, drove people away: dismembered bodies from Chacoan times were found at two sites in the canyon, and at Cowboy Wash in south-western Colorado there are traces of a violent attack around 1125–1150 that may have involved cannibalism; it may have been driven by starvation during the drought or by hostility towards newcomers from the Chuska Mountains. The Chacoan great houses, however, show little sign of defence, and the interpretation is disputed.

The people did not vanish. Chaco's influence continued at Aztec, whose buildings date to the 12th and 13th centuries, at Mesa Verde and in the Chuska Mountains, while others moved into the valleys of the Little Colorado, the Rio Puerco and the Rio Grande; archaeologists date the beginning of the Pueblo III period to about 1150. The end of Chaco has become a classic case in the study of societal collapse: Joseph Tainter made it one of the three case studies of The Collapse of Complex Societies (1988), alongside the Western Roman Empire and the Classic Maya, explaining such collapses by declining returns on investment in social complexity, with droughts and other shocks as only the apparent causes. The Hopi and other Pueblo peoples regard Chaco as an ancestral homeland and keep accounts of their migrations from it.