Assessing Hohokam Canal Stability through Stratigraphy

Prehistoric canals are common in the alluvial valleys of south-central Arizona and provide a means of understanding the region’s prehistory. Stratigraphy within the canals records operational dynamics and may be used to test the hypothesis that a period of increased flooding played a pivotal role in the Hohokam collapse in the 15th century. In this study, I analyze particle-size data from 45 prehistoric main canals located in three large systems, in the lower Salt River Valley. Overall canal sedimentology is similar among the three systems, and Markov chain analysis demonstrates that repeated depositional sequences are rare. This finding may be due to stratigraphic unconformities or a sampling strategy that emphasizes thick strata. In any event, unequivocal sedimentological evidence for uncontrolled flooding outside of flood plains is uncommon. Either floods were not the problem that the historical record suggests or flood deposits are poorly preserved and difficult to identify sedimentologically. A flood stratigraphy model for canals is needed to improve identification of prehistoric flooding and better recognize instability in former irrigation systems.