The impact of fluvial processes and landscape evolution on archaeological sites and settlement patterns along the San Xavier reach of the Santa Cruz River, Arizona

Abstract

The Santa Cruz arroyo in the Tucson Basin, Arizona, has undergone major environmental changes over the last 8000 years. The Holocene stratigraphy along a 15 km segment of the arroyo, known as the San Xavier reach, which traverses the Sen Xavier Indian Reservation was investigated in detail. The Holocene alluvial sequence reveals that aggradation occurred until 8OOO yr B.P. within a braided stream. was followed by a major period of channel erosion and widening from 8000 to 6600 yr B.P., which, in turn, was followed by vertical aggradation of the floodplain and five ahort periods of channel cutting. Broad climatic changes are correlated with major changes in the fluvial regime and landscape. However, the cycles of armyo cutting and filling, during the semiarid climate of the last 2500 years. were probably the result of the creation of unstable internal geomorphic conditions, flooding, and human impacta on the floodplain. The alluvial history of the San Xavier reach has had a pronounced effect on the preservation and completenesa of the archaeological record. Periods of erosion have created absences or gaps in the archaeological record and deep burial has removed some of the preserved archaeological remains from view. Environmental changes on the floodplain also influenced late prehistoric Hohokam settlement and subsistence patterns in the San Xavier region.

Temporal frequency distributions of alluvium in the American Southwest: taphonomic, paleohydraulic, and demographic implications

“…The oldest radiocarbon dates in our Santa Cruz sample (w19,200 BP and w22,000 BP) are from carbonates that occur in Quaternary-age braided stream deposits that underlie the Holocene-age inner valley alluvium. These ages extend the Santa Cruz chronology beyond 7970 AE 130 14 C yr BP (w8800 cal yr BP), the earliest radiocarbon date on organic matter from the San Xavier Bridge site (Waters, 1988) and the inner valley alluvium in general (Waters and Haynes, 2001). It is apparent that our alluvial data generally do not closely mimic the volcanic data.…”

“…Middle and early Holocene sediments are rare in the Santa Cruz Valley, but they also suffer significant discovery bias. Indeed, the earliest Holocene deposits in both inner valleys are deeply buried (Ballenger, 2010a;Mabry, 2006a;Waters, 1988). It is encouraging that the earliest archaeological carbon in the Santa Cruz Valley coincides with the earliest well-sampled geological sediments, possibly indicating a richer middle Holocene archaeological record than is apparent (Surovell et al, 2009).…”

“…A decline in archaeological radiocarbon dates relative to geological dates between 0 and 2000 cal yr BP in the Santa Cruz sample is at odds with regional demographic reconstructions that predict large populations around 1000 cal yr BP (Dean et al, 1994), but corresponds with a period when widespread channel entrenchment appears to have brought about significant changes in Hohokam floodplain farming practices and settlement (Waters, 1991;Waters and Ravesloot, 2000), and when ceramic typologies are favored for age control. The positive correlation observed between geological and archaeological radiocarbon dates during the Late Archaic/Early Agricultural period in the Santa Cruz data and the Clovis period in the San Pedro data can be explained by a combination of redundant archaeological and geological sampling and the occurrence of archaeological deposits near stratigraphic boundaries, conditions that are satisfied in both cases (Haynes, 2007b;Waters, 1988;Mabry, 2008).…”

“…We assembled a dataset of 717 14 C determinations from the arroyo channel fill, fans, and floodplains of the upper San Pedro (e.g., Ballenger, 2010a;Cook, 2007;Haas and Haynes, 1975;Haynes, 2007b;Haynes and Haas, 1974;Haynes et al, 1966Haynes et al, , 1967Haynes et al, , 1971Hopkins, 2010;Huckell, 1990;Pigati et al, 2008) and middle Santa Cruz Valleys (e.g., Freeman, 2000;Haynes and Huckell, 1986;Mabry, 2008;Waters, 1988). In addition to the published literature, we have included all of the radiocarbon dates contained in the index card catalog of C. Vance Haynes, Jr.…”

“…In cut-and-fill systems, the completeness of the geological and archaeological records is partly related to the duration and magnitude of erosional events (Waters, 1991). Arroyo cutting removes sediments that necessarily pre-date the onset of erosion; so discernibly fragmented chronologies are expected to occur immediately before and during cutting episodes (Waters, 1988). The time depth removed by erosion theoretically scales to the duration of erosive conditions (non-deposition) and erosional depth, the latter being controlled by base level and floor lithology (Bull, 1997;Leopold and Bull, 1979).…”