Drainage initiation, expansion, and channel-head arrest in heterogenous bedrock landscapes of the Colorado Plateau

Abstract

The transition between hillslopes and channels defines landscape characteristics such as drainage density, ridge spacing, and hillslope length. Previous studies of the location of this transition have demonstrated that soil advection and diffusion work either individually or in concert to set the limits of channelization in largely soil-mantled landscapes. However, no such model explaining the limits of channelization in bedrock landscapes has been established. In this study, we explore the morphologic signatures and processes of channel initiation, headward elongation, and eventual arrest in the bedrock landscape of the Raplee Ridge monocline, Utah, USA. The monocline provides an opportunity to test whether a threshold shear stress controls the initiation of channels through inspection of slope and upstream drainage area at varyingly incised portions of the landscape. Using a combination of lidar and structure-from-motion data, we find that incised and unincised channels occupy overlapping but separable portions of slope-upstream area space, while geomorphic mapping and field-based observations allow for further distinction between erosional processes with threshold shear stress values that range from ∼60 Pa to 160 Pa. We develop a conceptual model of the initiation and expansion of these bedrock channels using field observations, in which channels first initiate by vertical plucking of blocks, disaggregate limestone by block sliding, elongate upslope by knickpoint retreat in variably erosive lithologies, and eventually arrest at predictable landscape positions due to block buttressing. These results suggest that shear stress-controlled processes play a fundamental role in setting the degree of channelization in bedrock landscapes; however, the thresholds for channel erosion can change in response to local geologic factors such as lithology and structural geometry. This implies that detailed geologic knowledge may be necessary for the interpretation and modeling of fluvial channels in terrestrial bedrock landscapes and those on other planets.