Micromorphological study of irrigation canal sediments: Landscape evolution and hydraulic management in the middle Gila River valley (Phoenix Basin, Arizona) during the Hohokam occupation

The Hohokam were a long lived (300–1450 AD) agrarian society in the deserts of southern Arizona with an efficient irrigation system and a complex socio-political organization. Dependent on environmental conditions, they coped with hydrological and geomorphic changes, which potentially had an impact on their cultural evolution. Irrigation canals were regularly maintained and/or abandoned and their fills record secondary signatures of fluvial dynamics, landscape change and land use. Micromorphological studies allow for the creation of a typology of canal fills based on semi-quantitative measurements. A synchronous and diachronic analysis of canal systems can then be undertaken by means of pedosedimentary correlation. As part of cultural resource management projects on the Gila River Indian Reservation, south of Phoenix, studies have been conducted on the prehistoric Santan Irrigation System, in use from the 10th to the 15th century AD. Phases of low flow were identified, followed by alluvial fan flooding, and seasonal deposition between the 10th–14th century AD (Sedentary/Early Classic Period). During the 14th–15th century AD (Late Classic Period), evidence of active erosion is initially recorded in the sediments, followed by seasonal water supply, and ultimately canal abandonment despite probable efficient irrigation.