Early irrigation systems in southeastern Arizona: the ostracode perspective

For the first time, the Early Agricultural Period (1200 BC-150 AD) canal ir rigation in the Santa Cruz River Valley, southeastern Arizona, is documente d through ostracode paleoecology. Interpretations based on ostracode paleoe cology and taphonomy are supported by anthropological, sedimentological, ge omorphological, and palynological information, and were used to determine t he environmental history of the northern Tucson Basin during the time span represented by the sequence of canals at Las Capas (site AZ AA: 12:753 ASM) . We also attempt to elucidate based on archaeological artifacts if the Hoh okam or a previous civilization built the canals. Between 3000 and 2400 radiocarbon years BP, at least three episodes of cana l operation are defined by ostracode assemblages and pollen records. Modem (mid-late 20th century) canals supported no ostracodes, probably because of temporally brief canal operation from local wells. Three stages of water m anagement are well defined during prehistoric canal operation. Ostracode fa unal associations indicate that prehistoric peoples first operated their ir rigation systems in a simple, 'opportunistic' mode (diversion of ephemeral flows following storms), and later in a complex, 'functional' mode (careful ly timed diversions of perennial flows). The geomorphological reconstruction indicates that these canals had a minim um length of 1.1 kin, and were possibly twice as long. The hydraulic recons truction of these canals suggests that they had similar gradients (0.05-0.1 %) to later prehistoric canals in the same valley. Discharges were also res pectable. When flowing at bank-full, the largest canal provided an acre-foo t of water in about 2.3 h; when flowing half-full (probably a more realisti c assumption), it produced an acre-foot of water in about 8.6 h. Palynological records of the oldest canals (here identified as Features 3 a nd 4; 3000-2500 years BP) indicate they were used temporarily, since ripari an vegetation did not grow consistently in the area. The presence of maize (Zea sp.) pollen in the canals confirms agricultural use of the canal water . However, a low percentage of maize and weed pollen suggests limited agric ultural activity in this location, consistent with the lithostratigraphy, g ranulometry, and ostracode paleoecology. Agricultural fields were probably located downstream of this site. Ostracode assemblages show patterns consistent with the opportunistic or fu nctional water control method, hence proving their value as indicators of h uman activity and environmental change. The transition from opportunistic t o functional modes of canal operation indicates the increasing complexity o f the social structure in the Santa Cruz Valley during the San Pedro Phase (1200-800 BC) of the Early Agricultural Period. (C) 2001 Elsevier Science L td. All rights reserved.