How does lithology influence fluvial erodibility? A study of knickpoint retreat in small bedrock rivers on the Isle of Jura

Abstract

Bedrock rivers are a key driver of landscape evolution across large parts of Earth's continents. Though empirical, theoretical and numerical modelling efforts predict that bedrock erosion should be influenced by lithology, and more specifically, rock strength, relating rock strength to fluvial erodibility (K) in the field, has proven challenging. One recent suggestion as to why this may be the case is that fluvial erodibility is not controlled solely by bedrock strength, but instead the contrast in strength between the underlying bedrock and the strength of the impacting sediment carried by the river (bed load). The goal of this study is to examine how best rock strength measurements can be incorporated into estimates of fluvial erodibility. We do this by studying knickpoint migration, which is related to fluvial erodibility, on the Isle of Jura, SW Scotland. The Isle of Jura is an ideal field site as it hosts a suite of coeval knickpoints, associated with base level fall following glacio-isostatic adjustment, that have migrated over two separate rock types of differing strengths. We quantify rock strength on the island using the Schmidt hammer. We then predict knickpoint response times based on different models of fluvial erodibility informed by our rock strength data. The best fitting model of fluvial erodibility accounts for the contrast between bedrock and bed load strength. Interestingly, the model that simply assigns a different fluvial erodibility value for each lithological unit performs worse than a model that does not account for lithology. This finding challenges present practices for incorporating lithology in landscape evolution models.