Dynamique des paysages agraires et gestion de l'eau dans le bassin semi-désertique de Phoenix, Arizona de la Préhistoire à l'époque moderne

Abstract : The Hohokam and their descendants the Akimel O'Odham have cultivated and irrigated the lower Salt river valley and the middle Gila river valley in the semi-desert Phoenix basin in Arizona for almost two thousand years. The cultural history of this tribe is based on the continuous interaction of its members and water, a rare resource in the region. To fully comprehend the socio-environmental dynamics resulting from this interaction over a long period, we have opted for a geoarcheological, chronological and paleoenvironmental approach to hydraulic systems and alluvial formations, together with precise geophysical and micromorphological methods. The reconstruction of the agro-system and hydro-system brings to light a development in Hohokam irrigation technology around 300 AD in stable environmental conditions. From 300 AD to 1050 AD (pre-Classic period) the Hohokam agro-system demonstrates efficient irrigation of the alluvial plain and low terraces whose level rises then stabilises between 850 AD and 1000 AD (Medieval Climate Anomaly). There follows a widening period followed by an incision between 1000 AD and 1150 AD. In spite of considerable efforts to maintain the hydraulic structures, the land has to be reorganised. The lower lands are abandoned, new irrigation systems are built, and there is more agricultural diversity. As from the 13th century, there is another rise in the level of the alluvial plain and between 1150 AD and 1450 AD (Classical Period) there is a peak in irrigation efficiency. However, the Phoenix basin is abandoned until the 17th century. This land abandonment brings about a stability in the landscape between 1450 AD and 1600 AD and is followed by a rapid return to aggradation together with a strong fluvial mobility lasting until 1870 (Little Ice Age) at which time another incision is recorded. The Akimel O'Odham agro-system works with optimal results from the 17th century to the arrival of pioneers (circa 1870), following which it evolves in an almost irreversible way (reduction of irrigated areas, clearing of trees and dry farming practices). These results allow us to debate not only about the practical details of morphogenesis in semi-arid conditions but also about the concepts of socio-environmental stability and breakdown. We refute the hypothesis of environmental decline around 1450 AD, yet an environmental crisis around 1050-1150 AD, combined with difficult climatic conditions (fast climate change, El Niño Events), leading to massive population migrations and land overexploitation, might have caused a slow decline in the Hohokam irrigation community almost 200 to 200 years later. The socio-environmental system of the Akimel O'Odham in the 19th century seems to reproduce the same pattern on a smaller spatial and temporal scale.
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Submitted on : Saturday, May 5, 2012 - 10:28:58 AM
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Louise Purdue. Dynamique des paysages agraires et gestion de l'eau dans le bassin semi-désertique de Phoenix, Arizona de la Préhistoire à l'époque moderne. Etudes de l'environnement. Université Nice Sophia Antipolis, 2011. Français. ⟨tel-00694627⟩

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