Changes in river floodplain morphology can have devastating consequences for irrigation agriculturalists. Channel erosion occurred in the late nineteenth century, on the flood plain of the middle Gila River, Arizona and severely impacted the native Akimel O' odham (Pima) farmers. Prior to the Akimel O' odham, the prehistoric Hohokam also pursued irrigation agriculture along this river. Geoarchaeological investigations of the Gila River flood plain document a major period of channel cutting and widening sometime between A.D. 1020 to 1160. This channel erosion is coincident with the partial abandonment of large Hohokam villages and significant population rearrangements. It also marks the beginning of a major social reorganization when ball-courts were replaced by platform mounds as the social integrative structure and the Hohokam sphere of influence contracted. Other rivers utilized by the Hohokam—the Santa Cruz River, San Pedro River, and Tonto Creek-also experienced channel cutting between A.D. 1050 and 1150. Thus, a regional episode of channel erosion appears to have been a major factor that contributed to the reorganization seen in the Hohokam archaeological record. These synchronous landscape changes would have severely impacted Hohokam irrigation systems and food production capabilities. This undoubtedly created stresses within Hohokam society which in turn may have accelerated social, political, economic, ideological, and demographic changes that were already underway.

Stratigraphy and dating provide an 18,000 yr record of erosion and deposition by the middle Gila River. Prior to 18,000 cal yr B.P., the Gila River cut into its floodplain and created a deep, wide channel. Sand and gravel accumulated within this channel until 4250–4400 cal yr B.P. By this time, the channel of the Gila River was narrow and deep. Around 5000 cal yr B.P., fine-grained sediments began to accumulate on the floodplain. At 800–950 cal yr B.P., there was a major period of channel widening. After this erosional episode, the channel again narrowed as it filled with sand and gravel until around 200 cal yr B.P. On the floodplain, deposition continued from about 5000 cal yr B.P. to 500 cal yr B.P. A period of stability and soil formation occurred on the floodplain between 500 and 200 cal yr B.P., after which overbank deposition resumed and buried the soil. Channel widening again occurred in the late 19th century. The changes observed in the sedimentologic history of the Gila River show that during the late Pleistocene, the Gila River was a competent stream capable of carrying and depositing coarse sediment loads. Later deposition of sand and gravel during the first half of the Holocene implies an increase in sediment yield from upstream watersheds. Changes during the last 4000 years reflect the response of the river to climatic perturbations, the timing of large floods, internal landscape thresholds, and human impacts.

The prehistoric Hohokam conducted a great variety of activities in the spaces between their villages, including social gatherings and ceremonial observances as well as economic and subsistence practices. Recent full-coverage pedestrian survey along the middle Gila River in central Arizona indicates that nonresidential sites are more numerous and cover considerably greater area than residential settlements. Unfortunately, in the Hohokam region researchers are not always able to distinguish residential sites from activity areas based on features observable on the surface. In this study, quantitative measures of artifact density and diversity of surface collections from artifact scatters are used to distinguish residential sites from nonresidential sites. This is accomplished by assessing the extent to which their assemblages resemble artifact collections from known habitations, campsites, or specialized or diverse activity loci. Differences in artifact density and diversity enable many ambiguous artifact scatters to be classified into these general functional site types. Knowing the distribution of site types relative to elements of the natural and cultural landscapes can provide insight into past social and ecological or economic behaviors not offered by site-specific approaches. The study concludes that considering both the physical and cultural dimensions of landscapes significantly increases the research value of nonresidential sites for understanding the use and meaning of spaces between villages.

Eolian sediments are common within the middle Gila River Valley, southern Arizona, and reflect variability in eolian and fluvial processes during the late Holocene. This study focuses on deciphering the stratigraphic record of eolian deposition and associated luminescence dating of quartz extracts by single aliquot regeneration (SAR) protocols. Stratigraphic assessment coupled with luminescence ages indicates that there are four broad eolian depositional events at ca. 3145 ± 220 yr, 1950–1360 yr, 800 ± 100 yr, and 690–315 yr. This nascent chronology, correlated with regional archeological evidence and paleoclimate proxy datasets, leads to two general conclusions: (1) loess deposits, transverse-dune formation and sand-sheet deposition in the late Holocene are probably linked to flow variability of the Gila River, though the last two events are concordant with regional megadroughts; and (2) the stability of eolian landforms since the 19th century reflects the lack of eolian sediment supply during a period of fluvial incision, resulting in Entisol formation on dunes. The prime catalyst of eolian activity during the late Holocene is inferred to be sediment supply, driven by climate periodicity and variable flow within the Gila River catchment.