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.
San Juan River (Colorado River tributary)
The San Juan River and its tributaries were an important water source for Native Americans as early as 10,000 BC, when Paleo-Indians inhabited the Four Corners region. By 500 BC–450 AD, the Basketmaker culture was succeeded by the Ancestral Puebloans or Anasazi who developed distinctive irrigation methods and masonry architecture ("pueblos"); many ruins and sites are preserved in the San Juan watershed in places such as Mesa Verde National Park. Most settlements were concentrated along the upper San Juan River in New Mexico, where the terrain is gentler and water more abundant. The lower San Juan River in Utah flows through inaccessible canyons that largely precluded habitation. Starting around 1300 AD, a warming climate brought long droughts to the area, forcing the Puebloans to abandon their settlements north of the San Juan River, and eventually causing them to migrate out of the San Juan River basin to the Rio Grande Valley, where their descendants live today.