Interplay of sediment supply, river incision, and channel morphology revealed by the transient evolution of an experimental bedrock channel

In transient landscapes, adjustments in river channel width, roughness, and alluvial cover, in addition to slope, provide potentially important but poorly understood mechanisms by which bedrock channels accommodate changes in external forcing. We used a laboratory flume to investigate experimentally how bedrock channel slope, width, roughness, alluvial cover, and incision rate collectively adjusted during the transient incision of an initially smooth channel with a varying bed load supply rate. When the channel was free of alluvial cover, incision was focused over a fraction of the bed width that varied strongly with both bed load supply and bed load transport capacity. Nondimensionalization yields a relationship for the width of active incision that explicitly incorporates bed load supply rate, sediment grain size, and bed shear stress, which suggests that in natural channels, width may respond dynamically to accommodate changes in bed load sediment supply. Because increases in sediment supply widened the band of active bed load sediment transport and thus the width over which incision took place, mass removal from the bed scaled with sediment supply when the bed was free of cover, consistent with incision being limited by the availability of erosive tools. However, bed roughness growth due to the spatial variation of incision during the experiment eventually inhibited bed load transport efficiency. This, in turn, led to deposition of alluvial cover and the suppression of incision on the bed at high sediment supply rates, consistent with incision being limited by the extent of alluvial cover deposited on the bed. The dynamics of roughness creation and alluvial cover deposition can therefore drive both negative and positive feedbacks on incision rate change following sediment supply perturbations. These experimental results offer several potentially field-testable hypotheses that together may help explain variability in the width, slope, and bed roughness of bedrock river channels in transient landscapes.