Why Did You Leave Me? Identification of a Two-Phase Fluvial Incision Response to Climate, Land-Sse and Tectonic Changes Leading to Strath Terrace Formation
Résumé
Surpassing thresholds between different erosion regimes following abrupt changes in external forcing have the potential to shift rivers dramatically from one characteristic state to another. The alternating cycle of strath planation and strath terrace abandonment due to variations in sediment supply relative to the transport capacity is a common feature in many mountainous environments, yet our understanding of the mechanics of the processes that drive this landscape change remains poorly quantified.
Here, we present an experimental laboratory study that identifies the response of alluvial rivers to variable sediment supply, discharge and tilting, proxies for climatic, land-use and tectonic changes. The experimental channels exhibit a two-phase response: the narrowing of the channel and stripping of the alluvial cover in a downstream migrating incision wave followed by further focussing of the flow, destabilisation of the bed and development of knickpoints mid-channel under supercritical flow conditions, in the absence of any base level fall. Once formed, in the absence of any base level fall, the headward erosion by knickpoints is the most efficient process of strath terrace abandonment, with the majority of the total vertical incision occurring as the knickpoint migrates upstream. The resulting landscape state is characterised by a narrow deeply incised channel in strong contrast with the initial wide, laterally migrating system present before the perturbation was forced during the experiments.
These experiments demonstrate the importance of self-formed knickpoints in driving the rapid response of fluvial systems to external perturbations, highlighting a previously unrecognised process leading to strath terrace abandonment. This has implications for the understanding of distributions of strath terrace ages derived from geochronological techniques, and how rivers will respond to future perturbations driven by climate or land-use changes.