Knickpoint Retreat: the Role of Channel Self-Organisation
Résumé
Landscape connectivity occurs through the valley network, and the action of rivers within these valleys controls both landscape morphology, and landscape response to tectonic or climatic change. Such response to transient forcing is manifested in bedrock river profiles through migrating ‘knickzones’ or ‘knickpoints’, that separate a downstream reach, broadly in equilibrium with the new conditions and an upstream reach which is yet to adjust. Knickpoints therefore mark a dynamic boundary location within mountain landscapes, yet the complexities of the mechanisms of knickpoint retreat are often ignored in studies of landscape evolution.
We carried out a series of box flume experiments (65 cm long, 30 cm wide), to explore the importance of mean discharge and substrate strength on the form and migration of knickpoints in a cohesive homogenous substrate. The retreat rate of knickpoints is independent of mean discharge, with knickpoints retreating faster through a weaker substrate. Despite an order of magnitude increase in discharge during our experiments, the bed shear stress remains almost constant due to a self-regulatory response of channel width to higher discharge, leading to no change in the upstream retreat rate of the waterfalls.
These experiments challenge the established assumption in models of landscape evolution that a simple relationship exists between knickpoint retreat and discharge/drainage area, and we hypothesise that the correlation between knickpoint retreat and drainage area identified in some landscapes is caused by increasing bedload flux (the ‘tools’ for erosion) with drainage area. Knickpoint retreat modelling approaches should therefore be re-evaluated, and greater attention paid to the role of bedload flux and connectivity between river channels and hillslopes.