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.
Highlights
Knickpoints are discrete zones of steepened bed gradient that are commonly observed in river longitudinal profiles
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
We show that knickpoint initiation at the outlet coincides with a fairly abrupt river narrowing entailing an increase in their shear stress
Summary
Knickpoints are discrete zones of steepened bed gradient that are commonly observed in river longitudinal profiles They occasionally occur due to changes in bedrock properties (e.g., Duvall et al, 2004), in many cases they are dynamic features that propagate upstream along drainage networks (Whipple and Tucker, 1999; Kirby and Whipple, 2012; Whittaker and Boulton, 2012). Most observations of autogenic knickpoint formation come from experimental modeling (see for example Paola et al, 2009), their initiation being attributed to the amplification of local instabilities in flume (Scheingross et al, 2019) and drainage basin-scale (Hasbargen and Paola, 2000) experiments In these latter experiments, for example, successive knickpoints initiated despite constant external forcing
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