BIOGEOMORPHOLOGY AND SOIL GEOMORPHOLOGY OF SMALL SEMI-ARID BASINS, NORTHEASTERN ARIZONA: INFLUENCES OF TOPOCLIMATE AND CLIMATE VARIATION

Relatively small-scale Holocene climate changes have strongly influenced landscape evolution in the dryland areas of the Colorado Plateau. Most previous studies have focused on channel behavior and cycles of arroyo formation; few directly addressed the critical contributions of basin hillslopes -- a major source of runoff and sediments -- to basin floor channel responses. Soil geomorphic and dendrogeomorphic studies of hill slopes in a study area in northeastern Arizona on the southern Colorado show that slope responses to climate change are strongly influenced by rocktype, as certain types of sandstones rapidly weather by processes such as hydration of smectitic cements. Small changes in water availability in the resulting soil mantle can greatly influence the magnitude of the formation and erosion of these weathered materials. This ultimately promotes the development of two dominant slope forms mainly related to aspect-induced topoclimatic variation: (1) mesic, soil-mantled, transport-limited slopes with northern aspects; and (2) more xeric, generally steeper largely detachment-limited bedrock slopes with southern aspects. Our recent studies show that the composition of plant assemblages on the two aspects also differs in key ways, and so the end-member slope forms are also recognized as end-member ecological systems in this region. The environmental gradients, or zones of transition, between adjacent ecological systems are defined as ecotones. On the basis of previous research in the study area that indicates that periods of greatly increased erosion from soil-mantled slopes correlate with climatic shifts from multi-year droughts to sustained periods of above average precipitation over the past five hundred years, we hypothesize that these and previous climate changes in the Holocene have caused, and are likely causing, the transition of the topoclimatic and ecological state on the mesic hillslopes to an increasingly more xeric ecological state. The geomorphic consequences of these changes have been changes in vegetation assemblages, soil development, slope-forming processes and ultimately the largely irreversible evolution of dominantly soil-mantled slopes to bedrock-dominated slopes.