Articles | Volume 4, issue 1
Research article
15 Jan 2016
Research article |  | 15 Jan 2016

Experimental migration of knickpoints: influence of style of base-level fall and bed lithology

J.-L. Grimaud, C. Paola, and V. Voller

Abstract. Knickpoints are fascinating and common geomorphic features whose dynamics influence the development of landscapes and source-to-sink systems – in particular the upstream propagation of erosion. Here, we study river profiles and associated knickpoints experimentally in a microflume filled with a cohesive substrate made of silica, water and kaolinite. We focus on the effect on knickpoint dynamics of varying the distribution of base-level fall (rate, increment, and period) and substrate strength, i.e., kaolinite content. Such simple cases are directly comparable to both bedrock and alluvial river systems. Under a constant rate of base-level fall, knickpoints of similar shape are periodically generated, highlighting self-organized dynamics in which steady forcing leads to multiple knickpoint events. Temporary shielding of the bed by alluvium controls the spacing between these unit knickpoints. Shielding is, however, not effective when base-level drops exceed alluvium thickness. While the base-level fall rate controls the overall slope of experiments, it is not instrumental in dictating the major characteristics of unit knickpoints. Instead the velocity, face slope and associated plunge pool depth of these knickpoints are all strongly influenced by lithology. The period between knickpoints is set by both alluvium thickness and base-level fall rate, allowing use of knickpoint spacing along rivers as an indicator of base-level fall rate.

Short summary
Knickpoints represent localized steps along a river profile (e.g. waterfalls or rapids) that are commonly interpreted as the geomorphic response of river systems to external changes. We used a simple experiment to show that knickpoints may not only respond to external base-level change but are also able to self-organize. We highlight the effect of alluvial cover in delaying knickpoint formation and show that river bed strength controls both retreat velocity and geometry of knickpoints.