Knickpoint Morphology in Homogenous Substrates Controlled by Sediment Cover and Base‐Level Fall Rate

ABSTRACT

Knickpoints are prominent geomorphic features (the most well‐known being waterfalls) defined as the transition between upstream relict topography and downstream topography adjusted to boundary conditions. Despite being localised within rivers (10s of m), knickpoints profoundly control landscape evolution in the basin where they are located. In contrast to knickpoint retreat rates, studies assessing knickpoint morphology are sparse. Using systematic flume experiments, this study observes knickpoints routinely forming in homogenous‐substrate channels under constant forcing (i.e., base‐level fall rate and sediment flux). We demonstrate for the first time that (i) these knickpoints can have three distinct morphologies and (ii) their morphology is controlled by the balance between base‐level fall rate () and sediment flux (). Unstable conditions are created when outpaces vertical incision () of the channel (i.e., ). Channel steepening occurs due to erosion perpendicular to the knickpoint face at a shear stress maxima downstream of the steepest point. Thus, high base‐level fall rates where there is a greater difference between and are conducive to steep knickpoint slopes. Sediment armouring most readily occurs at the knickpoint base, reducing the erosion rate relative to the lip. This reduces knickpoint slopes when the base is well protected. When sediment flux is low, vertical knickpoints are maintained as plunge pools develop and erode upstream. The deposition of eroded plunge pool material downstream of the knickpoint armours the channel and enhances the difference between and . This study demonstrates that changing morphology may increase or decrease knickpoint erosional effectiveness through changes to its resultant erosion vector. More widely, this study represents a starting point to develop a process‐based understanding of how sediment flux and base‐level fall rate impact knickpoints in longitudinal river profiles providing the potential for more accurate models of landscape evolution across a variety of settings.