Agriculture in the prehistoric Southwest
After the Hohokam disappeared, Spanish explorers of the 16th century noted the use of canal irrigation in only two areas of the Southwest: eastern Sonora, primarily practiced by the Opata and Lower Pima, and among the Pueblos of northern New Mexico. Irrigation by the Opata and Pueblos had different objectives. In Sonora, with a long growing season, two crops of maize a year were grown in river valleys. The spring crop during the dry season was dependent upon irrigation; the summer and fall crop during the rainy season utilized irrigation to supplement rainfall. The necessity of irrigation probably required a high level of social organization. In New Mexico, with only one crop of maize per year possible, irrigation was supplemental and the lesser dependence on irrigation implies less social organization than in Sonora.
Chetro Ketl
As the Basketmaker III people improved their farming techniques during the 8th century, the well-watered areas of the San Juan Basin became densely populated. Greater crop yields necessitated the construction of above-ground storage facilities, which were the first large-scale construction projects in the region. Fagan identifies this as the beginning of the first pueblos, "ushering in a period of profound social tension, population movements, and political change." In his opinion, "by 800, there was no going back. The people of Chaco and elsewhere were locked completely into economies based on maize and bean cultivation." Archeologists refer to the period starting as the Pueblo I Period. By the early 10th century the large pit-house settlements had been supplanted by modular construction that later served as the foundation for the Ancestral Puebloan great houses. This marks the beginning of the Bonito Phase. During the 10th century, Chaco's population was swelled by a steady influx of immigrants from the San Juan River, approximately north. Fagan notes, "Within a few centuries, as rainfall became more irregular and life less predictable, the Chacoans embarked on a cultural trajectory that melded ancient traditions with new ideas that were to crystalize into a brilliant and short-lived Southwestern society."