ABSTRACT
Quaternary fluvial geomorphic adjustments on three spatial and temporal scales have taken place in Chaco Canyon, in scmiarid northwestern New Mexico. These adjustments are reflected in the morphology and sedimentology of Chaco Arroyo, and in the alluvial canyon fill and terrace deposits above the modern canyon floor. Facies preserved within the deposits, coupled with historical and archaeological data, aid in interpreting the processes of the fluvial adjustments in a temporal framework. Increases in stream power transport sediment beyond the canyon and/or modify the type of sediment deposited within the canyon. Decreases in stream power allow aggradation within the canyon.
The smallest fluvial adjustments include ephemeral changes within the modern arroyo and along the canyon margins. The development of Chaco Arroyo during the past 140 yrs is related to changes in precipitation, to changes in land management, and to inherent fluvial adjustments.
Intermediate adjustments include wide-spread alluviation of the canyon floor interrupted by cycles of cut-and-fill. The morphology and sedimentology of channel cuts and fills and the facies of the alluvial canyon floor indicate that there are (1) contrasting processes of channel formation, (2) different means of filling channels, and (3) that thin sheets of local- and headwater-derived sediments are intercalated when the canyon floor has no entrenched channels. The results of an evaluation of the influence of intrinsic and extrinsic variables on cycles of cut-and-fill are somewhat equivocal. Land use does not appear to have been a major influence on channels in Chaco Canyon, even during the extensive occupation of the canyon nearly 1,000 yrs ago.
The greatest adjustments in Chaco Canyon were induced by long-term cycles of erosion and deposition during which fluvial processes eroded alluvium from the canyon, cut the bedrock floor, and partially refilled the canyon with alluvium. Deposits indicating large, long-term fluctuations in discharge are preserved along the margins of the canyon at several topographic levels. Gravel deposits indicating stream power much larger than present form terraces and morphologically indistinct mounds. Deposits indicating a278 hydrologic regime similar to the present are preserved by carbonate cementation in low-energy environments along the canyon margins. Alternation of sediment types suggest that there are geomorphic thresholds and complex responses even at this scale of adjustment. Long-term climatic fluctuations are responsible for alternations between these two types of fluvial regimes.
