Culture and Ecology of Chaco Canyon and the San Juan Basin

262 Chaco Project SynthesismtJes such as Shabik'eshchee Village and PeachSprings. Gillespie and Powers suggested that theremay have been a change in subsistence that emphasizedincreased dependence on cultivated plants,especially at slightly lower locations. The beginningof San Juan Basin communities, such as those at GreyRidge, Peach Springs, and Kin Ya'a, occurred duringthis period. In the Rio Puerco of the east, greatersummer precipitation and runoff from the tributariesfrom Mount Taylor, Mesa Chivato, and the NacimientoMountains would have permitted increased useof a lower elevation zone, 1,646 to 1,829 m.During Pueblo II, the Rio San Jose became thefourth major subregion based on the number of sitespresent. Substantial regionwide popUlation growthwas noted. Existing communities increased in sizeand new ones, often with Chacoan structures, wereestablished. Much of the growth took place in the1,829 to 2,195 m zone, but settlement was noted inhigher and lower elevation zones. Climatically, theperiod was thought to have had wetter conditions(A.D. 950 to the early A.D. 1100s), with increasedsummer rainfall and an expansion of summermonsoons. This would allow for expansion into newareas during periods with longer frost-free seasons.Gillespie and Powers noted general agreement betweenpeaks in summer precipitation and building constructionepisodes (A.D. 91Os, 945-953, 970s, andA.D. 1034 to 1080) at several of the Chaco Canyongreat houses. They attributed this increase in part todemographic growth and pressure during this period,with favorable temperatures and moisture regimes thatallowed use of higher and lower elevation zones.Social and economic changes would have accompaniedthese increases, especially after mobility becamesomewhat circumscribed; thus, regional exchange andirrigation systems would have been instituted.During Pueblo III, there are two distinct periods:From A.D. 1100 to 1130, there was a continuedbuilding frenzy, especially in the San Juan-Animasriver area (which became an important populationcenter), the central basin, and Chaco Canyon. Yet byA.D. 1150 to 1175, many of the sites-and even entirecommunities-had been abandoned, probably in relationto the collapse of Chaco Canyon. The continueduse of the San Juan-Animas area through the thirteenthcentury was attributed to the availability of water inthe perennial streams that could have been used forirrigation. During this period the greatest reduction insite numbers occurred in elevation zones above 2,195or below 1,829 m. This change was attributed to a50-year decrease in summer rainfall levels (Rose et al.1982), with possibly cooler temperatures occurring inthe twelfth and thirteenth centuries, which would haveaffected agriculture below the 1,829 m level. Althoughthere was not a complete migration out of theSan Juan Basin, there was a major migration to themargins of the basin, the northern river valleys, andthe southeast highlands. (See also Stuart and Gauthier1980, who discuss the widespread adaptation tohighlands at this time.)In summary, Gillespie and Powers deciphered ageneral correlation between climatic data-especiallyrainfall patterns; the use of different elevation zonesfor farming; and the growth and decline of the Chacosystem. This initial correlation would be refined byseveral investigators; the most detailed such study isthat of Gwinn Vivian (1990). He thought that ChacoCanyon had some advantages over the surroundingregion due to its unique features within the ChacoBasin, where numerous and more closely spaced sidedrainage systems collect more concentrated rainfallthat could have been used for crop production.As noted in chapter 4, Gwinn Vivian (1990) indicatedthat two different architectural and settlementpatterns were present in the Chaco core by Pueblo I.By A.D. 920, Vivian's Rosa variant to the northeastof the canyon was oriented toward a highland adaptation,and it shows little interaction with the Chacocore. On the southern periphery of the San JuanBasin, the White Mound-Kiatuthlanna variant expandedto encompass some of the Lorna Alta on theeast. In the La Plata-Piedra variant, representative ofwell-established farmers from the north, there wasconstruction of larger sites similar to those that wouldbecome the great houses that mark the Bonito phase,while the White Mound-Kiatuthlanna variant lived insmall sites similar to those found in the southern SanJuan Basin. The overlapping boundaries of thesevariants in and around Chaco Canyon suggested somemeans of integration of these distinct groups (seebelow).Gwinn Vivian (1990) discussed the Classicperiod in three segments, based on dates used by theChaco Project. The Early Bonito phase, characterized