Channel stability in steep gravel-cobble streams is controlled by the coarse tail of the bed material distribution

Researchers have associated channel-forming flows with reach-average shear stresses close to the entrainment threshold for the surfaceD(50). We conducted experiments using a model of a generic steep, gravel-cobble stream to test this association. Our results suggest that channel-forming flows fully mobilize theD(50), and produce shear stresses close to the entrainment threshold for the largest grains in the bed. The channel dimensions were set by flows capable of mobilizing between 85% and 90% of the bed surface, which produced a brief period of lateral instability lasting about 1 h, followed by a prolonged period of relative stability during which modest adjustments occurred, but during which the reach-average hydraulics remained about the same. The adjustments during the unstable phase of the experiments are characterized by rapid bank erosion, extensive deposits on the channel bed and a restructuring of the major morphologic elements of the stream. The adjustments during the stable phase of the experiments involved barform migration and bed surface coarsening but did not appreciably modify the physical template established by the end of the unstable phase. The behaviour we observed is not consistent with the concept of a dynamic equilibrium associated with a formative flow that is just capable of entraining the bed surfaceD(50). Instead, it suggests that rapid adjustments occur once a stability threshold is exceeded, which creates a template that constrains channel activity until another event drives the system across the stability threshold, and re-sets the template. While we believe that it is probably too simplistic to associate a channel-forming discharge with the entrainment threshold for a single grain size, our results suggest that theD(95)is a more logical choice than theD(50)(c) 2020 John Wiley & Sons, Ltd.