Combined field and novel laboratory constraints on river bedrock erodibility

Numerical models of mountain evolution commonly assume that bedrock river incision rate is proportional to shear stress on the channel and lump the influence of rock strength and other factors, including those related to climate, into an erodibility term. The erodibility term is critical for understanding changes in fluvial relief and the timescale of mountain landscape response to changes in environmental forcings. Isolating the influence of lithology is a major challenge necessary to improving the predictive ability of models and our understanding of river incision. Rock strength exerts a first order control on a erodibility by controlling how rock fractures and erodes, affecting bedrock incision rates and topography. Outstanding problems in bedrock river incision include the control of lithology on bedrock erodibility through its bulk strength, fracturing, bedload generation and transport.

To constrain the lithological control on bedrock incision and erodibility, we connect observations at the river reach and laboratory scale. We have designed a new abrasion-mill-type device for quantifying the control of rock strength and fracturing on erosion rates. The laboratory device is designed to generate saltating grains which can be tracked to quantify the energy of impact, and allows us to test natural rock samples without the need for drill cores. We combine our experimental abrasion results and measurements of mechanical properties of rock samples, with field observations of bedload character and river channel geometry in a natural river to constrain the controls of lithology and rock strength on erodibility. Our results will be of use to field geomorphologists and numerical modelers who are interested in modelling or quantifying rates of erosion in mountainous settings.