Autogenic knickpoints in laboratory landscape experiments

Abstract

Abstract. The upstream propagation of knickpoints in river longitudinal profiles is commonly assumed to be related to discrete changes in tectonics, climate or base level. However, the recognition that some knickpoints may form autogenically, independent of any external perturbation, may challenge these assumptions. We investigate here the genesis and dynamics of such autogenic knickpoints in laboratory experiments at the drainage basin scale, where landscapes evolved in response to constant rates of base level fall and precipitation. Despite these constant forcings, we observe that knickpoints regularly initiate in rivers at the catchments' outlet throughout the duration of experiments. The upstream knickpoint propagation rate does not decrease monotonically in relationship with the decrease in drainage area, as predicted by stream-power-based models, instead the propagation rate first increases until the mid-part of catchments before decreasing. To investigate the dynamics of the knickpoints, we calculated hydraulic information (water depth, river width, discharge and shear stress) using a hydrodynamic model. We show that knickpoint initiation at the outlet coincides with a fairly abrupt river narrowing entailing an increase in their shear stress. Then, once knickpoints have propagated upward, rivers widen causing a decrease in shear stress and incision rate, and making the river incision less than the base level fall rate. This creates an unstable situation which drives the formation of a new knickpoint. The experiments suggest a new autocyclic model of knickpoint generation controlled by river width dynamics independent of variations in climate or tectonics. This questions an interpretation of landscape records focusing only on climate and tectonic changes without considering autogenic processes.

Dynamic bedrock channel width during knickpoint retreat enhances undercutting of coupled hillslopes

“…Swiss Alps- Korup & Schlunegger, 2007;Southern Spain-Reinhardt et al, 2007;Southern Appalachians-Gallen et al, 2011;Kaua'i, Hawai'i-Mackey et al, 2014;California-Hurst et al, 2019), as well as in laboratory experiments (e.g. Bigi et al, 2006;de Lavaissiere et al, 2022;Hasbergen & Paola, 2000). In contrast, the importance of processes that govern lateral adjustment of bedrock channel geometry has received relatively little attention compared to vertical incision processes (e.g.…”

Tectonic advection of contacts enhances landscape transience

“…This possibility could be significant, as other studies have proposed the maintenance of topographic relief in the Appalachian Mountains through lithospheric delamination or changes in mantle dynamics (Gallen et al, 2013; Miller et al, 2013) or subsidence within the Gulf of Mexico and western Atlantic Ocean driving flexural uplift and tilting of the Appalachians (Liu, 2014). Additionally, lateral channel migration can trigger landslides, and such autogenic perturbations can drive landscape erosion and relief over long periods (Campforts et al, 2022; de Lavaissière et al, 2022; Hasbargen & Paola, 2000; Scheingross et al, 2020). Are novel and complex dynamics required to explain landscapes like the Appalachians, or can the internal dynamics of landscapes themselves offer sufficient explanations?…”

Impacts of Spontaneous Waterfall Development on Bedrock River Longitudinal Profile Morphology

“…However, not all knickzones form due to external perturbations of base level. Knickzones can form due to lithologic heterogeneity where rivers cross locally resistant rock (e.g., Forte et al., 2016; Frankel et al., 2007; Gabet, 2019; Holland & Pickup, 1976; Mitchell & Yanites, 2019; Perne et al., 2017), and have also been argued to be emergent features that can manifest in homogeneous lithology and in the absence of climatic and tectonic perturbations (Baynes et al., 2018; de Lavaissière et al., 2022; Hasbargen & Paola, 2000; Scheingross et al., 2020). However, this self‐emergent (i.e., autogenic) mechanism has seen comparatively little study.…”