Abstract

Bed load transport during storm events is both an agent of geomorphic change and a significant
natural hazard in mountain regions. Thus, predicting bed load transport is a central challenge in fluvial
geomorphology and natural hazard risk assessment. Bed load transport during storm events depends on the
width and depth of bed scour, as well as the transport distances of individual sediment grains. We traced
individual gravels in two steep mountain streams, the Erlenbach (Switzerland) and Rio Cordon (Italy), using
magnetic and radio frequency identification tags, and measured their bed load transport rates using calibrated
geophone bed load sensors in the Erlenbach and a bed load trap in the Rio Cordon. Tracer transport distances
and bed load volumes exhibited approximate power law scaling with both the peak stream power and the
cumulative stream energy of individual hydrologic events. Bed load volumes scaled much more steeply with
peak stream power and cumulative stream energy than tracer transport distances did, and bed load volumes
scaled as roughly the third power of transport distances. These observations imply that large bed load transport
events become large primarily by scouring the bed deeper and wider, and only secondarily by transporting the
mobilized sediment farther. Using the sediment continuity equation, we can estimate the mean effective
thickness of the actively transported layer, averaged over the entire channel width and the duration of
individual flow events. This active layer thickness also followed approximate power law scaling with peak
streampower and cumulative stream energy and ranged up to 0.57min the Erlenbach, broadly consistent with
independent measurements.