22.04.2015: In mountain regions rivers often cut deep gorges into bedrock and in this way drive the topographic evolution of the landscape. Propelled by the power of the flowing water, particles jump, roll or slide over the rock and abrade material from the bed. Mechanistic models of fluvial bedrock erosion are based on the assumption that erosion rate is dependent on the energy delivered to the bed by the impacts of moving pebbles. These models, however, do not capture the influence of the size of the pebbles, but work with a single, representative particle size.

To study how erosion rate is influenced by particle size, the GFZ scientist Jens Turowski and his colleagues Alexander Beer and Carlos Wyss from ETH Zürich/WSL combined methods to measure both the grain sizes transported by the river and the energy delivered to the bed. The results were startling: the largest size class with a median diameter of 86 mm contributed only 8.8% of the total mass, but delivered 41.5% of the energy to the bed. This disproportionate energy delivery by large stones is not captured by existing models. A reason for the discrepancy could be that the models treat only jumping stones, while rolling or sliding motion is neglected. The latter two modes of motion can be most commonly observed with large particles.

The new results complicate the modelling of bedrock erosion by rivers, and thereby the evolution of entire landscapes. The grain size distribution on the bed of a mountain stream is determined mainly by the material delivered from hillslopes, making it necessary to better understand the connection between hillslopes and stream channels. But since the largest stones are only transported in rare extreme floods, modelling efforts could be focused on such exceptional events.

Turowski, J.M., C.R. Wyss, A.R. Beer, 2015, Grain size effects on energy delivery to the stream bed and links to bedrock erosion, Geophysical Research Letters, 42, 1775-1780, doi: 10.1002/2015GL063159