Culture and Ecology of Chaco Canyon and the San Juan Basin

Environment and Natural Resources 31approximately 5.08 to 15.24 cm (2 to 6 in) per yearbetween 1934 and 1972-a period when grazing withinthe park was eliminated and agriculture was notpracticed. However, planting of vegetation during the1930s may have trapped sediments eroding from thearroyo walls and could be interpreted as alluviationafter the 1920s (DeAngelis 1972), but Dean (personalcommunication, 2000) indicates that in other areaswhere no vegetation was planted the same eventsoccurred. The vegetation did slow the stream flowsand encourage sediment deposition; it indirectlyretarded vertical erosion, because water had lessvelocity to scour or undercut the vertical banks.Recently, Kirk Vincent from USGS (Brad Shattuck,personal communication, 2002) suggested that thearroyo was not deeper but rather went from a sand-bedchannel with a vertical arroyo wall to a vegetativeplain with an inner channel. The former sand channelbecame a new floodplain.Other documentation of changes in the recentfloodplain exists in photographs taken by Chauvenet(1935) and Malde in 1972; Malde (2001) continues toupdate this photographic record. Malde's 1970sinvestigation established transects selected to representvegetational types defined in Loren Potter's (1974)studies, as well as provide data on soil erosionaccumulation and bank-cutting studies. To accomplishthe latter, scour chains and erosion pins were installedand monitored (Simons, Li & Associates 1982).Although relatively little net change occurred over aperiod of four years, there were differences betweenwashes. For example, Fajada Wash showed thehighest net aggradation (35 cm) while the Gallo Washhad an 8 cm net degradation over a five-year period.Rates of surface lowering ranged between 1 to 4mm/yr. At the higher elevated canyon floor nearFajada Butte, the rate was 0.2 mm/yr; in contrast, therate was 9 mm/yr where transect B3 crossed an area ofactive gullying and piping adjacent to Fajada Wash.When the Chaco erosion rates were compared to thosefrom other semiarid and badland areas, the resultswere relatively similar (Simons, Li & Associates1982:3.43-3.45).In summary, human actions alone cannot be themajor cause for changes described in the Chaco Wash.DeAngelis (1972) proposed that overgrazing andclimatic fluctuations were complementary, rather thanmutually exclusive, causes for arroyo formation. Ifarroyo-cutting occurs when the gradient of the alluvialfill of a restricted valley increases until it is no longerstable and channel trenching begins when the criticalangle is reached, overgrazing, aridity, and climaticfactors must interact to lower the critical anglenecessary for arroyo-cutting. Even small-scaleclimatic changes would affect the level of fill withinthe Chaco Wash; intermediate-scale changes wouldalter the character of the canyon bottom such that theremight be a flat floor or an entrenched arroyo. Duringany short period, Pueblo inhabitants would haveencountered somewhat different conditions than theirancestors or descendants did. The construction of amasonry dam across the Chaco Wash at its confluencewith the Escavada Wash, probably in the mid-eleventhcentury, would have raised the water table in thecanyon (Force et a1. 2002) and brought advantages tothe agriculturalists. Archaeologists have not yet hadtime to evaluate this information, but future researchon this topic will enhance our understanding of Pueblouse of the canyon. Until recently, researchers weredependent on climatic data to guide their interpretationsof this cultural period.ClimateThe last major climatic change occurred at theend of the Pleistocene as a warming trend began. Butwithin the Holocene, how much change and preciselywhen it occurred have been topics of research fornumerous scholars. In early studies of the Puebloadaptation in Chaco, rainfall was considered a keyvariable that affects the diminution of plant cover anderosion of soils (e.g., Bryan 1954; Fisher 1934;Hawley 1934; Hewett 1936; Judd 1954, 1964).Beginning in 1932, a weather station was set up atPueblo Bonito, but data collected included only theamount of precipitation and maximum and minimumtemperatures.In an initial evaluation of climate, Fisher (1934)used data from these and other stations in the San JuanBasin to suggest that killing frosts ended the secondweek of May and began around the first week inOctober; he estimated a ISO-day growing season.Fisher realized that data on fluctuations in rainfall inChaco Canyon were unavailable, but that new treeringstudies would provide approximate precipitationvalues by year.