Bedrock river erosion through dipping layered rocks: quantifying erodibility through kinematic wave speed

Nate A Mitchell,Brian J Yanites

doi:10.5194/esurf-9-723-2021

Abstract

Abstract. Landscape morphology reflects drivers such as tectonics and climate but is also modulated by underlying rock properties. While geomorphologists may attempt to quantify the influence of rock strength through direct comparisons of landscape morphology and rock strength metrics, recent work has shown that the contact migration resulting from the presence of mixed lithologies may hinder such an approach. Indeed, this work counterintuitively suggests that channel slopes within weaker units can sometimes be higher than channel slopes within stronger units. Here, we expand upon previous work with 1-D stream power numerical models in which we have created a system for quantifying contact migration over time. Although previous studies have developed theories for bedrock rivers incising through layered stratigraphy, we can now scrutinize these theories with contact migration rates measured in our models. Our results show that previously developed theory is generally robust and that contact migration rates reflect the pattern of kinematic wave speed across the profile. Furthermore, we have developed and tested a new approach for estimating kinematic wave speeds. This approach utilizes channel steepness, a known base-level fall rate, and contact dips. Importantly, we demonstrate how this new approach can be combined with previous work to estimate erodibility values. We demonstrate this approach by accurately estimating the erodibility values used in our numerical models. After this demonstration, we use our approach to estimate erodibility values for a stream near Hanksville, UT. Because we show in our numerical models that one can estimate the erodibility of the unit with lower steepness, the erodibilities we estimate for this stream in Utah are likely representative of mudstone and/or siltstone. The methods we have developed can be applied to streams with temporally constant base-level fall, opening new avenues of research within the field of geomorphology.

Highlights

Geomorphologists seek to extract geologic and climatic information from landscape morphology, and the conceptual framework of the stream power model (Howard and Kerby, 1983; Whipple and Tucker, 1999) has driven many such endeavors (Whipple et al, 2013)
We show here that for bedrock rivers incising through layered rocks, rock strength contrasts between different units can alter channel slopes, erosion rates, and kinematic wave speeds along the river profile
That the influence of rock strength contrasts is a predictable phenomenon across a range of contact dips and erodibilities

Summary

Introduction

Geomorphologists seek to extract geologic and climatic information from landscape morphology, and the conceptual framework of the stream power model (Howard and Kerby, 1983; Whipple and Tucker, 1999) has driven many such endeavors (Whipple et al, 2013). As a representation of bedrock river incision, the stream power model has been used in many applications, including (1) identifying unrecognized earthquake risks (Kirby et al, 2003), (2) constraining the timing and extent of normal fault activity (Whittaker et al, 2008; Boulton and Whittaker, 2009; Gallen and Wegmann, 2017), (3) distinguishing between potential drivers of transient incision (Carretier et al, 2006; Gallen et al, 2013; Miller et al, 2013; Yanites et al, 2017), and (4) searching for spatial patterns in rock strength (Allen et al, 2013; Bursztyn et al, 2015).

Methods

Results

Discussion

Conclusion