Supercritical River Terraces: Processes driving rapid landscape change following climatic variability

International audienceThe alternating cycle of strath planation and strath terrace abandonment due to variations in sediment supplyrelative to river transport capacity is a common feature in many mountainous environments, often driven by climaticvariability. However, our understanding of the mechanics of the processes that drive this cycle remains poorlyquantified. Here, we used an experimental and numerical study to identify the geomorphic and hydraulic controlsdriving the response of alluvial rivers to variable sediment supply, discharge and slope. The experimental channelsexhibit a newly identified multi-stage response: the narrowing of the channel and stripping of the alluvial coverin a downstream migrating incision wave followed by destabilisation of the bedrock surface and development ofself-formed knickpoints when the hydraulic conditions are supercritical. Headward erosion by knickpoints is themost efficient process of strath terrace abandonment, contributing the majority of the total vertical incision, even inthe absence of base-level fall or any vertical offset in channel elevation.We also demonstrate the possibility of selfformedknickpoints developing under supercritical flow conditions in driving the rapid response of fluvial systemsto external climatic perturbations in natural systems, highlighting a previously unrecognised process leading tostrath terrace abandonment. This has implications for the understanding of distributions of strath terrace ages, andhow landscapes respond to climatic or tectonic perturbations