Culture and Ecology of Chaco Canyon and the San Juan Basin

~--------------------- --24 Chaco Project SynthesisFirst, the Chaco Wash is not a true subsequentstream. Although it may have developed along thestrike of a nonresistant bed, the present wash was mostlikely imposed upon more resistant formations.Second, stream capture for several tributarieswas unusual. Based on the extent of the subbasins andangular junctions of the Coyote (draining 2,281 km 2or 881 mi 2 ), Escavada (draining 587 km 2 or 227 mi2),and Fajada (draining 523 km 2 or 202 mi 2 ) washes,they may represent three actual headwaters that werecaptured into a single wash through time. At eachjunction, the Chaco Wash makes a nearly 90-degreechange in course, whereas the tributary wash appearsto represent a natural headward extension of the mainChaco channel. D. Love (1977b: 12) also questionedwhether the Escavada or the Chaco was the masterstream for the Chaco River, and he recommendedfurther study to determine the reasons forentrenchment of the Chaco-Escavada junction intobedrock. At the time, the implications of changes atthis junction were not understood, but recent studiesby Force et al. (2002) propose that breaching of a sanddune dam during the Pueblo adaptation would haveaffected ground water levels and water availability inthe lower canyon.Third, although the longitudinal profile of theChaco Wash is typical of an ephemeral stream, astream gradient anomaly occurs in Chaco Canyonbetween 1,859 and 1,920 m (6,100 and 6,300 ft)contours. Within a distance of 22 km (13.7 mi)between these contours (the main canyon, where manylarge Pueblo sites are located), the stream gradientaverages 4.48 mper 1.6 km (14.7 ft per mi), which isconsistent with gradients farther upstream anddownstream. The gradient of the uncut alluvial floorin that distance, however, averages 5.79 m per 1.6 km(19 ft per mi). The 0.91 to 1.2 m per 1.6 km (3 to 4ft per mi) difference in gradients was thought to reflectthe effects of alluviation in the canyon between A.D.1300 and 1860. The significance of this anomalycould be understood through review of processes ofarroyo formation. Whether this anomaly holds truefor earlier periods was not known. Knowing thecauses of alluviation and the periods when such eventsoccurred affects interpretations of human use of thecanyon.That the Chaco provided evidence for more thanone series of erosional and depositional events waspreviously documented (Bryan 1941, 1954; Dodge1920; Jackson 1878). Based on Bryan's more detailedstudies of the strata that contained early Pueblosherds, he suggested three periods of dissection andalluviation in the canyon, with the last depositionoccurring in the twelfth century. He thought thatthese occurrences were not unique to this locality, butrather could be correlated with evidence from otherstream localities in the northern Southwest. Bryanalso postulated that the abandonment of the canyon byPueblo ancestors may have been due to progressiveupstream erosion of the wash due to climatic change;e.g., slightly decreased rainfall that led to arroyoformation. If erosional and depositional events werecontemporaneous over the larger region, they wouldhave affected the population throughout the San JuanBasin in a similar manner during each period. Each ofthese concepts was re-examined during the ChacoProject.Bryan (1941, 1954) suggested that the oldestperiod of deposition was probably Pleistocene in agedue to the presence of extinct faunal remains in thealluvial fill; it would have had no effect on humanoccupation. The first period of erosion was very long;Bryan thought it indicated climatic change, but it wasdifficult to date. Although there was very littleevidence to suggest human occupation during thisperiod, silt from one of the horizons was thought tocorrelate with the Archaic period or the Cochiseculture. The second depositional period was not welldated. It might represent two episodes separated by aninterval of erosion; and sherds present in the fillranged from around A.D. 300 through 1250. Thenext erosional episode in Chaco Canyon was thoughtto occur after A.D. 1167 but prior to about A.D.1250. Sherds in the fill of other streams indicate thatdeposition began before or shortly after A.D. 1400.The present erosional cycle was dated around A.D.1860 to 1890, depending on the area. Although therewas a lack of more exact dating in seven dispersedareas, Bryan concluded that these cutting and fillingepisodes were widespread rather than canyon-specific.The restricted area where DeAngelis noted the0.91 to 1.22 m per 1.60 km difference in gradients