Culture and Ecology of Chaco Canyon and the San Juan Basin

Explaining Pueblo Social Organization 273Of the environmental stimuli (erosion, decreasedlength of growing season, increased salinity, andwater availability), water availability in a semiaridenvironment was considered the most likely stimulusfor trophic change. Such stimuli could evoke a changeto a higher niche initially in a local area (a positiveeffect) and cause collapse if it was an area-wideproblem (a negative effect) (Judge 1976b, 1979).Tree-ring studies used to model precipitation patternswere under way (Dean and Robinson 1977; Robinsonand Rose 1979; Rose 1979; Rose et al. 1982), andchanges in culture were evaluated against fluctuationsin rainfall patterns.Based on a number of studies, including pollencounts, both Judge (1976b) and Gwinn Vivian (1990)recognized an environmental change, which causedtrophic change, during the middle A.D. 800s.Variability in available moisture suggests thatprominent below-normal rainfall periods occurred inthe A.D. 920s through 940s; declining values in theA.D. 1080s and 1090s, with a minor positive peak inthe late A.D. 1080s; above-average values in the earlyA.D. 1100s; a drought between A.D. 1130 and 1180;improVed rainfall in the early A.D. 12oos; and lowvalues in the last three decades of the A.D. 1200s(Gillespie and Powers 1983; Powers et al. 1983:279-283, Figures 145 and 146; Schelberg 1982a). Severedrought from A.D. 1130 to 1180 would have had amajor effect on agriculturalists on the southernColorado Plateau (Gillespie 1985).During periods of negative rainfall, suggestedinvestments in energy subsidies included 1) the importationof foodstuffs, which could be measured throughincreases in storage capacity, and 2) intensification ofagriculture, which would be evident through imprOVedcrop species, increased production, and technologicalchange (Judge 1979). The earliest great houses, withlayouts similar to contemporary small site units butwith much larger rooms, were thought to haveprovided increased storage area (Lekson 1984a;Lekson et al. 1988). Later McElmo-style greathouses, with their unit pattern of one circular structureenclosed by numerous rectangular rooms, and similarunit additions to Bonito-style great houses, werethought to represent storage space (Lekson 1984a;Lekson et al. 1988).Improvement in crop species is difficult to dis-cern. Although M. Toll (1985) documented changesin cob size, number of rows of com, and cupule size,good data from Chaco Canyon came from only onegreat house and two small sites. When compared withother sites in the San Juan Basin, she noted that somechange may be due to differences in available waterrather than species improvements.Increasing food production could be achieved byincreasing the total area under cultivation or the yieldper acre. Increasing the total acreage under cultivationwould be limited by slope, soil type, and availablewater. In Chaco Canyon, the canyon bottom was bestsuited for agriculture, and the areas with the greatestamount of available water are confluences of sidedrainage systems with the Chaco Wash. If onlyrainfall was available, terracing of other areas withoutwater control would probably have been inadequate.Improved overall moisture after ca. A. D. 900 woulddecrease the risk of farming in marginal areas throughoutthe Chaco Basin and allow outward expansion(Judge et al. 1981). It was assumed that duringPueblo II, productive lands in the canyon were filled;thus, expansion would stop, and technologicalimprovements would be needed (Judge 1977a).The most evident technological improvement isconstruction of dams and canals taking water frommesa tops to gridded gardens (Gwinn Vivian 1972).Although Judge (1977a) considered the Chaco inhabitantscapable of constructing a water control system,as described by Vivian (1970b, 1972, 1974b) by A.D.850, the masonry style and most of the associatedceramics recovered from test trenches in severalsystems suggested the existence of a formalized systemaround the middle A.D. 1000s and early A.D. lloos.The few Pueblo I sherds noted by Loose and Lyons(1976a) in the bottom layer of the Chetro Ketl fieldhinted at possible early use, but their provenienceassociation was questionable.Families that utilized better watered garden plotswould develop suprafamily cooperation units to maintainthe system as it evolved. Pooling and redistributionof resources within local communities wouldoccur. Construction of the earliest great houses (UnaVida, Pueblo Bonito, and Penasco Blanco) occurred atthe confluence of m~or side drainage systems (F~adaWash, South Gap, and the Escavada Wash, respectively)with the Chaco Wash. Their much larger and