Archaeological implications of late‐holocene channel changes on the middle gila river, arizona

Archival and stratigraphic data from the middle Gila River, Arizona, provide insight into late‐Holocene floods and channel changes that affected the Hohokam, prehistoric agriculturalists who practiced canal irrigation 2000‐500 B.P. The relationship between floods and channel changes during the past 300 years is characterized by an increased frequency of wide, braided channel conditions during episodes of increased large flood frequency. Ten radiocarbon ages from overbank deposits from the middle Gila River flood plain indicate that the frequency of large floods was relatively low 4000‐1000 B.P. ( 14 C years) but increased after 1000 B.P., a pattern supported by paleoflood slackwater sites in the upper Gila River watershed. This suggests that channel instability (shifts between narrow, single‐channel and wide, braided planforms) increased after 1000 B.P. The latter part of the 4000‐1000 B.P. period of relative channel stability corresponds in time with the development of Hohokam irrigation agriculture and expansion of canal systems, whereas increased fluvial dynamics beginning 1000 B.P. correlate (in time) to changes in settlement patterns during the Sedentary‐Classic period transition. Higher large flood frequency may be responsible for the consolidation of canal systems and relocation of villages along the middle Gila River after 1000 B.P. and during the Classic period. Increased flooding and concomitant channel changes alone, however, do not adequately explain the collapse of the Hohokam pattern.