Uncovering the Controls on Fluvial Bedrock Erodibility and Knickpoint Expression: A High‐Resolution Comparison of Bedrock Properties Between Knickpoints and Non‐Knickpoint Reaches

Abstract

AbstractBedrock erodibility exerts a fundamental control on fluvial incision, and therefore on the evolution of entire landscapes. However, the roles and relative significance of specific bedrock properties like rock strength and discontinuity spacing in setting erodibility remain poorly understood. As a result, erodibility is often overlooked or oversimplified in numerical and field‐based investigations of landscape evolution processes, leading to misinterpretation of channel profile convexities (i.e., knickpoints) in studies seeking to infer uplift or baselevel histories from longitudinal profile analyses. Here, we investigate the controls on fluvial bedrock erodibility and knickpoint expression by conducting detailed surveys of 21 lithologic knickpoints and non‐knickpoint reaches (representing end‐member bedrock erodibility cases) and corresponding bedrock properties from small channels in the south‐central Appalachian Mountains. We use binary logistic regression of field data to test the relative strength of rock strength and discontinuity spacing as predictors of knickpoint occurrence, and therefore their relative impact on fluvial bedrock erodibility. We find that discontinuity spacing more strongly influences bedrock erodibility in this setting, where both rock strength and discontinuity spacing vary widely, confirming for the first time quantitatively the hypothesis that discontinuities exert a dominant control on fluvial erodibility. We also find that knickpoint expression is unique to a given stratigraphic interval, implying that knickpoint morphology is intimately linked to local conditions and therefore effectively unpredictable without detailed field measurements. Finally, because all 21 of our study knickpoints occur within a single geologic unit, our results illustrate that intra‐unit heterogeneity must be accounted for when considering lithologic influence on channel profile convexities.