Understanding the impact of sediment supply, base-level fall rate and bedrock strength on knickpoint dynamics in fluvial environments.

Knickpoints are prominent geomorphic features, and despite being localized (i.e. 10s of m) within longitudinal river profiles they profoundly influence landscape evolution with impacts occurring both within the channel system and outside of it due to channel-hillslope interactions. Because of this, a thorough mechanistic understanding of knickpoint dynamics is imperative to understanding landscapes, yet the study of knickpoints remains limited. The long geological timescales over which knickpoints evolve in natural settings makes in-situ measurements of knickpoints challenging. The Bedrock River Experimental Incision Tank at the Université de Rennes 1 was used to perform analogue experiments to provide a comprehensive dataset assessing the impact of sediment supply, base-level fall rate and bedrock strength on knickpoint dynamics. Experiments were conducted with a homogenous lithology made up of a mixture of silica and glass beads with ratios ranging from 2.5:1 to 4:1. The mixture was combined with water and used to simulate bedrock, eroded by clear water flow, with the strength of the material set by the ratio of silica to glass beads. The experiments were run with constant water discharge (1.5 l min^-1), sediment supply ranging from 0 g l^-1 to 20 g l^-1,and base-level fall rates between 1.5 cm hr^-1 and 5 cm hr^-1. We find that knickpoint retreat exists between two end member states: knickpoint replacement and headward migration. It is illustrated that sediment supply impacts the channel’s ability to diffuse the knickpoint lip, whilst protecting the base, and base-level fall rate impacts the time the channel has to tend towards headward migration. Furthermore, we construct a phase diagram to illustrate the intricate interplay of sediment supply and base-level fall rate in shaping knickpoint morphology. Secondly, we find that lithology is the key determinant of knickpoint retreat rate where harder bedrock strengths result in lower knickpoint retreat rates, and channels respond to faster base level fall rates by generating more knickpoints rather than knickpoints retreating faster. This study provides a key insight into important controls on knickpoint morphology and retreat rate and provides a starting point for accurately modelling how knickpoint morphology varies as it migrates through a channel system.