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

• Published in: Journal of geophysical research. Earth surface Vol. 127; no. 3
• Main Authors: Chilton, Kristin D., Spotila, James A.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-03-2022
• Subjects: Bedrock; bedrock erodibility; discontinuities; Discontinuity; Erosion rates; Evolution; Fluvial morphology; Geologic units; Geology; Heterogeneity; knickpoints; Landscape; Lithology; Mathematical models; Morphology; Mountains; Numerical models; Properties; River channels; river incision; River morphology; Rivers; rock strength; Rocks; Stratigraphy; Strength; Surveys; Thickness; Uplift; Water discharge
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1029/2021JF006511

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.
Plain Language Summary
River incision into bedrock is a major driver of landscape evolution across most of Earth's terrestrial surface. The rate and pattern of this incision is strongly influenced by the erodibility of the underlying bedrock (defined as the rate at which erosion occurs for a given river slope and water discharge), which can vary significantly within a landscape. Currently, we have an incomplete understanding of the factors which influence bedrock erodibility and how it relates to river form and behavior. Therefore, the impacts of erodibility are not well captured by numerical models of river incision or adequately accounted for in field studies of river morphology. In this paper, we assume knickpoints (steep segments along a river's profile) are indicators of more resistant bedrock, and compare bedrock properties (e.g., sedimentary bed thickness and rock strength) between knickpoints and non‐knickpoint river reaches to infer which is a better predictor of knickpoint occurrence and therefore more impactful on bedrock erodibility. We find that within this setting, bed thickness is more important than rock strength in setting bedrock erodibility. We also observe that knickpoint morphology is fine‐tuned to local bedrock properties. Finally, because all of our study knickpoints occur within one geologic unit, studies seeking to interpret knickpoint distributions within a landscape must consider how heterogeneity within mapped geologic units may impact knickpoints within their study areas.
Key Points
Knickpoint occurrence, and therefore bedrock erodibility, is more strongly a function of bed thickness than rock strength in this setting
Knickpoint morphology is dictated by complex and unique combinations of local (sub‐meter scale) bedrock characteristics
Lithologic heterogeneity below the scale of mapped geologic units must be considered when interpreting spatial distributions of knickpoints