Culture and Ecology of Chaco Canyon and the San Juan Basin

26 Chaco Project Synthesisthe headwaters and/or side washes does not enter theChaco at the same time. During a rainstorm, watersfrom the local side channels often run earlier thanthose of the headwaters, if the latter runs at all.Because the timing of the appearance of such watersdiffered, during a major storm there was often no realincrease in discharge into the Chaco arroyo. Downcuttingseemed to be associated with an increase indischarge and stream power on readily erodiblematerial, and it seemed to end when the discharge wasadequately handled by the newly developed hydraulicgeometry of the new channel (Leopold and Miller1956). Love confirmed Hodges's (1974) opinion thatthere are a number of complex geomorphic variablesthat affected the Chaco Wash.Characteristics of sedimentary structures, color,grain size, grain mineralogy, clay mineralogy, andsoluble cations were studied to differentiate localenvironments. By incorporating Siemers's data, D.Love (1980) was able to distinguish clays from theMenefee Formation, Cliff House Sandstone, LewisShale, and Pictured Cliffs Sandstone by examinationof clay mineralogy. Unfortunately, after sediment istransported into the canyon, Love could not assign itto these specific types. He could, however, distinguishmost local deposits from headwater depositsbased on mineralogy and color (D. Love 1977b,1980).When he mapped and described the modemgeomorphic features of the Chaco Wash, D. Love(1983a, 1983b) listed the following: 1) bench, slope,and cliff topography on either side of the canyon floorrising up to 180 m above the canyon; 2) shortpediment talus fans at the base of the cliffs; 3)individual large rock falls; 4) side canyons cut into thecliffs on either side of the main canyon, which tend toparallel the regional jointing trend to the northeast,and which tend to be long on the southwest side andshort on the northwest side of the canyon; 5) alluvialfans at the mouths of side canyons and re-entrantsalong the main canyon; 6) sand dunes on tops of cliffsand mesas and in some of the side canyons; 7) silt-claydunes on the canyon floor; 8) flat canyon floor; 9)yazoo channels parallel to the main arroyo and sidearroyos; and 10) the Chaco arroyo (Figure 2.2).Piping is also present.Because his work was carried out between 1973and 1975, D. Love (1977a, 1977b, 1980) was able toprovide an in-depth description of the Chaco arroyoprior to and after major floods. These observations,combined with his other studies, suggested an outlinefor three scales of geomorphic adjustments in theChaco arroyo (D. Love 1979, 1983a). At each scale,changes in stream flow are involved.First, minor changes in precipitation werecorrelated with small-scale changes. These includeadjustments made within the past 140 years and arelimited to the inner channel and inner floodplain. Itwas evident on aerial photographs that the channel hadwidened extensively between 1900 and 1934, possiblydue to excess sediments or a sediment change. Withthe exception of a moist period from July 1940through December 1941 during which ThreateningRock fell on Pueblo Bonito, a decrease in moistureoccurred that lasted until the mid-1950s. Based on alongitudinal profile of the canyon floor, which wasnot broken by steep reaches, Love found no axial fansspread down the canyon floor. The arroyo meanders,with some loops pointing up canyon, indicating thatthe channel had been established on the floor prior todowncutting of the Chaco arroyo. This evidence didnot support Bryan's (1954) model, in which hepostulated that the arroyo formed by headward cuttingof a previously unchanneled alluvial canyon floor.There were several possible reasons why the channelchanged from a braided one to an inner channel andfloodplain. The adjustments are due in part to changesin precipitation, land management, and the inherentfluvial system. D. Love (1983a: 199) preferred toexplain channel development based on "changes indischarge, sediment load, and subsequent internaladjustments related to a period of less precipitationregionally. " The timing of the initial channel cut,however, was not as clear. Review of written reportsand interviews with local inhabitants suggested threepossibilities: 1) a pre-1849 date for the initial channelcut and partial fill; 2) an initial cut, partial fill, and reentrenchmentbetween 1849 and 1877; and 3) aninitial cut prior to 1877, with the presence of analluvial terrace from 1877 through the late 1880s, andthen re-entrenchment in the 1890s, with arroyowideningand removal of most of the alluvial terracebetween 1905 and the 1930s. Love favored the last,