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

Abstract

Bedrock 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.

One Million Years of Climate-Driven Rock Uplift Rate Variation on the Wasatch Fault Revealed by Fluvial Topography

“…To convert the Schmidt hammer rebound numbers into rock strength (here quantified as the Uniaxial Compressive Strength, UCS), we use equation ( 21), following Deere and Miller (1966) and Saptono et al (2013), where, Here, ρ is rock density in g cm −3 and R is the rebound value. We note that while rock abrasion rates experimentally scale with tensile strength (Sklar & Dietrich, 2001, other factors such as sediment supply (Sklar & Dietrich, 2004;Turowski et al, 2007), grain size, joint spacing (Chilton & Spotila, 2022) and the ratio between bedload and bedrock strength (Fox et al, 2023) influence erosion rates. Furthermore, uniaxial compressive strength and tensile strength are related to one another in Mohr-Coulomb space, thus we elect to describe rock strength using the more commonly reported uniaxial compressive strength metric.…”

Sediment cover modulates landscape erosion patterns and channel steepness in layered rocks: Insights from the SPACE Model

“…Quantifying the erodibility of different lithologies in the field remains an outstanding challenge in geomorphology (e.g., Chilton & Spotila, 2022;Moore et al, 2009;Shobe et al, 2020;Zondervan et al, 2020). Unlike bedrock erodibility, sediment properties are easier to quantify.…”

“…Climatic and tectonic signals are recorded in the profiles of bedrock river channels, which can be used to make inferences about these signals as they propagate through landscapes (Crosby & Whipple, 2006;Kirby & Whipple, 2012;Norton & Schlunegger, 2011;Whittaker, 2012;Wobus et al, 2006). However, distinguishing between climatic and tectonic effects on river channel evolution is hampered by our relatively limited understanding of lithology's influence on channel steepness and erosion rates (Chilton & Spotila, 2022;Gasparini & Whipple, 2014;Kirby & Whipple, 2012;Shobe et al, 2020). Lithology controls channel evolution in two main ways: by setting the erodibility of the channel substrate and by producing coarse sediment.…”

Sediment Cover Modulates Landscape Erosion Patterns and Channel Steepness in Layered Rocks: Insights From the SPACE Model

“…However, knickzones can also form due to changes in lithology. Distinguishing between climatic and tectonic effects on river channel evolution is hampered by our relatively limited understanding of lithology's influence on channel steepness and erosion rates (Chilton & Spotila, 2022; Gasparini & Whipple, 2014; Kirby & Whipple, 2012; Shobe et al., 2020). Lithology controls channel evolution in two main ways: by setting the erodibility of the channel substrate and by producing sediment.…”