Culture and Ecology of Chaco Canyon and the San Juan Basin

Environment and Natural Resources 27CHACO CANYONTERRAC£ INN£R F'LOOOPl.AIN INNER CHANNEl.. INNER F'lOOOPl,AINFigure 2.2. Geomorphic features of the Chaco arroyo. (Taken from Love 1979:Figure 3.)because it best fit the data. Thus, the smallest scaleadjustments took place within this one channel withina century and a half.Second, intermediate-scale fluvial adjustmentswere those that involved the aggradation of canyon filland cycles of channel cut and fill in three stages: a flatfloor, channel entrenchment, and channel fill until aflat floor was again present. When the floor is flat,alluvial fans from the canyon margin continue toaggrade and lenses of sand and clay spread across thefloor. Also, headwaters could spread from margin tomargin in narrow parts of the canyon during a floodthat ran at a rate of 125 m 2 per sec. To D. Love, thethinness of the deposits suggested that 1) either thecanyon commonly had a channel that confined theheadwater sediments; 2) a somewhat steady state oftransport of sediment through the canyon was achievedwhen no confining channel was present; or 3) largeflood events were rare, particularly when there was noconfining channel. Dating was least precise for theintermediate-scale changes, which encompass theperiod that includes the prehistoric use of ChacoCanyon, but several archaeornagnetic dates fell withinthis period (Nichols 1975). Three sets of archaeomagneticsamples from an upper laminated clay layerin the Chetro Ketl field suggested a date of A.D.1250. Twelve samples from two test trenches in ameander scar provided eight possible dates. Theuppermost sample, taken at 15.24 cm (6 in) below thesurface, suggested deposition around A.D. 1850.Four samples taken at depths from 38.1 to 68.5 cm