Water, rock and mountains: dissecting fluvial erodibility in the stream power model

Bedrock incision models, such as the stream power model, are powerful tools in modelling surface processes in response to changes in environmental forces including climate change. Such models often assume that the bedrock river incision rate is proportional to shear stress on the river channel and lump the influences of a number of factors, including rock strength and others related to climate, into a singular erodibility term. This erodibility term is critical for understanding changes in fluvial relief and the timescale of landscape responses to temporal fluctuations in external forcings. Simplifying numerous factors into one erodibility term can obscure the relative importance of change in bedrock strength, channel width scaling, sediment tools and climate in space and time. Therefore, dissecting the erodibility term is a major challenge necessary to improving the predictive ability of stream power models.

To dissect the erodibility term, we connect observations at the river reach and laboratory scale. We have designed a new abrasion-mill device for quantifying the control of rock strength and fracturing on erosion rates, while quantifying and keeping sediment impact energy constant. The laboratory device is designed to generate saltating grains which can be tracked to quantify the energy of impact, and allows us to fit natural rock samples with some degree of freedom in sample shapes. We combine our smaller-scale observations from experimental abrasion results and mechanical properties of the rock samples, with larger-scale observations of river channel geometry and slope in a natural river to develop a holistic framework of river incision. Our results will be of use to field geomorphologists and numerical modellers who are interested in modelling or quantifying rates of erosion in mountainous settings.