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.
Colorado River
These sedentary peoples heavily exploited their surroundings, practicing logging and harvesting of other resources on a large scale. The construction of irrigation canals may have led to a significant change in the morphology of many waterways in the Colorado River basin. Prior to human contact, rivers such as the Gila, Salt and Chaco were shallow perennial streams with low, vegetated banks and large floodplains. In time, flash floods caused significant downcutting on irrigation canals, which in turn led to the entrenchment of the original streams into arroyos, making agriculture difficult. A variety of methods were employed to combat these problems, including the construction of large dams, but when a megadrought hit the region in the 14th century A.D. the ancient civilizations of the Colorado River basin abruptly collapsed. Some Puebloans migrated to the Rio Grande Valley of central New Mexico and south-central Colorado, becoming the predecessors of the Hopi, Zuni, Laguna and Acoma people in western New Mexico. Many of the tribes that inhabited the Colorado River basin at the time of European contact were descended from Puebloan and Hohokam survivors, while others already had a long history of living in the region or migrated in from bordering lands.