Hohokam Canal Irrigation and the Formation of Irragric Anthrosols in the Middle Gila River Valley, Arizona, USA

Abstract

Irragric anthrosols form as a result of prolonged deposition of fine sediments from irrigation water. Ancient irragric soils centuries to millennia old occur in several world regions, especially in arid environments of Asia and the Americas. This article presents evidence for an ancient irragric anthrosol in the North American Southwest, along the Snaketown Canal System in the Middle Gila River Valley, Arizona. This pedostratigraphic unit was formed as a result of a millennium of irrigation by Hohokam farmers from A.D. 450 to 1450. The irragric soil consists of a mantle of silty‐to‐loamy textures with minimal soil formation overlying a natural argillic horizon on a Pleistocene stream terrace. A soil mapped independently by the United States Department of Agriculture‐Natural Resources Conservation Service with these horizons corresponds closely with the canal system. Soil within the canal system tends to be lower in salt, sodium, and pH compared with external soils. This suggests that the irragric process improved soil for crop production through long‐term leaching and additions of fresh sediments with the irrigation water. This anthropogenic process of canal sedimentation has had a long‐lasting impact on the sedimentary record and soils in this arid environment.

Selected highlights in American soil science history from the 1980s to the mid-2010s

“…Another study of the soil quality of irragric soils (anthrosols formed as a result of prolonged deposition of silty to loamy sediments from irrigation water that overlie a natural argillic horizon) was recently completed next to Snaketown, an ancient Hohokam village located near the Gila River in central Arizona that was occupied from~A.D. 450 to 1450 (Woodson et al, 2015). The Snaketown study corroborates some of the findings at the Las Capas site.…”

The geomorphic legacy of water and erosion control structures in a semiarid rangeland watershed

“…In the deserts of North America, runoff capture and redistribution of surface-water flow through irrigation systems enhanced prehistoric agriculture (Andrews and Bostwick, 2000). These water diversions were constructed over several centuries to irrigate agricultural fields, which resulted in distinct soil textures and chemistry that persists today (Woodson et al, 2015). Surface-water runoff was also captured and retained through the use of rock alignments forming contour terraces and grids that supported prehistoric agave cultivation.…”

Ancestral O'odham: Akimel O'ODHAM Cultural Traditions and the Archaeological Record

“…Because of these differences in the archaeological visibility of structures and household organization in different regions of the Southwest, any analysis that employs uniform room count assumptions for estimating population sizes will substantially underestimate the Preclassic- and Historic-period occupations in the Phoenix Basin. Therefore, the following discussion also incorporates several other proxy measures (e.g., public architecture size and irrigation system reconstructions) to estimate population densities over time (Fish and Fish 1994; Woodson 2016).…”