Hohokam Irrigation Longevity and Agricultural Success in the Lower Salt River Valley, Arizona

The primary focus of this research is the poorly understood relationship between water insufficiency and broad-scale social change, in the semi-arid lower Salt River Valley, in central Arizona. The overarching research question guiding this research is if water insufficiency could have prompted sociopolitical change among the Hohokam. Specifically, the research investigates if long-term water deficits were a catalyst for the two most consequential transformations in Hohokam history – the Preclassic/Classic transition (A.D. 1070-1100/1150) and the early to late Classic period transition (ca. A.D. 1300).

This research used extensive historical aerial photographs and cultural resource management excavation data to complete the largest-scale reconstruction of Hohokam irrigation. These lines of evidence provided exceptional insight into the developmental histories of eight major irrigation systems along the lower Salt River, four of which are newly defined here. Also, historic Salt River streamflow trends are leveraged to refine previously reconstructed annual flow discharges. The irrigation system reconstruction provided the means for estimating irrigation demand through irrigated acreage, and monthly streamflows supplied the amount of water available during key points in the two agricultural cycles per year. Together, irrigation demand and water availability provided necessary data to identify persistent water shortages during Hohokam history between A.D. 740 and 1450.

The findings discussed in this dissertation demonstrate that water insufficiency likely had no notable effect related to either the Preclassic/Classic or early to late Classictransitions in the lower Salt River Valley. Instead, there was possibly enough water through time for Hohokam farmers to meet agricultural demands. Three substantial additional insights were gained from this research. First, an extremely large flood, occurring either during the late Colonial or early Sedentary periods, may have profoundly altered irrigation agriculture and social organization in the valley. Second, during at least the Sedentary and Classic periods, Hohokam irrigation was structured into standardized irrigation units (SIU), a far more complex and efficient method of irrigation than previously perceived along the lower Salt. Third, a bedrock reef located near Canal System 2, and not at other lower Salt irrigation systems, is plausibly a determinate in Canal System 2’s longevity.