MORPHOLOGY AND PRESERVATION OF SHOULDER TYPE ESCARPMENTS CONTROLLED BY A FEEDBACK BETWEEN KNICKPOINT AND DIVIDE MIGRATION

Topographic escarpments are characterized by an extreme slope change across escarpment edge that separates a highland from a lowland, promoting escarpment retreat through time. Escarpments are typically divided into arch-type, where the drainage divide is located inland, and the knickpoints that flow across the escarpment can retreat and embay the escarpment, and shoulder-type, where the drainage divide aligns with the escarpment edge, which is expected to cause a slow and spatially uniform escarpment retreat. However, shoulder-type escarpments are sometimes associated with deep embayments that destruct the linear morphology and influence their long-term evolution. Yet, the processes that cause and sustain these embayments remain largely unexplored. We note that these embayments are commonly associated with reversed channels (i.e., highland channels that used to flow away from the escarpment along antecedent valleys, and currently flow toward and across the escarpment). Reversed channels typically develop over erodible lithology exposed within antecedent valleys, often underlaid by resilient rocks. We hypothesize that a positive feedback between knickpoint retreat and divide migration along such reversed channels, can potentially sustain the escarpment embayment. In this feedback, divide migration away from the escarpment increases the discharge along the knickpoint, facilitating knickpoint retreat which, in turn, changes the local base level for the divide and drives further divide migration. We constructed a one-dimensional numerical model to simulate this proposed feedback mechanism and explored the sensitivity of this feedback to lithologic and geomorphologic factors. We used this model, combined with topographic analysis, to study drainage divide migration along reversed channels in the Negev Desert, Israel, and show that divide and knickpoint migration in this area, and the embayments in the associated escarpments, can be explained by the proposed feedback mechanism. Our results suggest that this feedback may explain some of the global variability in escarpment morphology.