SEEPAGE, SEDIMENT SUPPLY, AND BEDROCK CHANNEL INCISION IN CANYONS OF THE ESCALANTE RIVER, UTAH

Tributary canyons of the Escalante River in southern Utah have previously been interpreted as key Martian analogues that suggest a groundwater-erosion origin for large valleys on Mars. This interpretation holds that fluvial processes are negligible, and that canyon cutting is instead dominated by headwall retreat at lithologic contacts caused by groundwater weathering and erosion. However, we suggest that observed patterns of canyon incision are significantly influenced by feedbacks between sediment supply and fluvial erosion. The observation that little fluvial incision has occurred into Navajo sandstone upstream of many headwalls may indicate that erosion is inhibited in sediment-starved channels because of a lack of sediment impacting and abrading the bed (the “tools effect" of the recently proposed saltation-abrasion model). Escalante tributary canyons provide a unique geological field setting to evaluate the tools hypothesis and aspects of other bedrock incision models because many of the controlling variables can be constrained. We focus on tributary reaches in the Navajo sandstone; this eolian unit forms massive cliffs but also readily weathers to sand, producing very little coarse sediment. However, preliminary field work has shown that the distribution of resistant upstream lithologies and sediment stored in upstream fill terraces results in variable amounts of coarse sediment supplied to different channels. Channel profiles determined from digital elevation model analysis and field observations point to dramatic differences in incision between channels. In addition to sediment supply effects, changes in channel morphology may also influence incision rates and cause much of the observed variability in longitudinal profiles. Extreme downstream changes in channel width and bed roughness, dramatically expressed as localized slot canyons, illustrate how bedrock channel morphology can adjust in complex ways.