Autogenic knickpoints in laboratory landscape experiments

[摘要] 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 suchautogenic 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 thatknickpoints regularly initiate in rivers at the catchments' outlet throughout the duration of experiments. The upstream knickpoint propagationrate does not decrease monotonically in relationship with the decrease indrainage area, as predicted by stream-power-based models, instead thepropagation rate first increases until the mid-part of catchments beforedecreasing. To investigate the dynamics of the knickpoints, we calculatedhydraulic 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.

[学科分类] 土壤学