Evolution of Shoulder Type Escarpments Through Feedback Between Knickpoint and Windgap Migration

Abstract

Escarpments and cliffs (hereafter termed escarpments) form topographic barriers that influence the spatial patterns of climate and biodiversity. The inherent extreme slope change across the escarpment edge promotes escarpment retreat over time. Typically, escarpments are divided into arch- and shoulder-types. In arch-type, the drainage divide is located inland, and knickpoints, located where channels flow across the escarpment, can retreat and embay the escarpment. In shoulder-type, the divide aligns with the escarpment edge, a setting expected to cause a slow and uniform escarpment retreat, preserving their integrity as barriers through time. However, observations from around the globe reveal shoulder-type escarpments are associated with deep embayments (i.e., alcoves) that destroy the linear appearance of the escarpment front. Yet, the processes that produce and sustain these embayments remain largely unexplored. Embayments of shoulder-type escarpments typically occur along reversed channels which were part of the antecedent drainage that used to flow away from the escarpment but now flow toward it, often resulting in a valley confined drainage divide called a windgap. Here, we hypothesize that feedback between knickpoint retreat and windgap migration away from the escarpment along reversed channels can sustain escarpment embayments, and use topographic analyses and numerical simulations to explore this hypothesis. Our analyses, focused on field sites in the Negev Desert, show that embayments of shoulder-type escarpments can be sustained through the hypothesized feedback, and quantify the sensitivity of this feedback to geomorphologic and lithologic parameters. Results suggest that this feedback may explain some of the global variability of escarpment morphologies.