Slope-dependent riverbed strengthening and the evolution of the threshold for motion in gravel-bed rivers

The threshold for motion exerts a fundamental control on sediment transport, channel morphology, and fluvial erosion. While channel slope and flow history are known to influence entrainment thresholds in gravel-bed rivers, their interaction has not been explored. We investigate how channel slope modulates riverbed strengthening during low-flow periods. Flume experiments were conducted across slopes spanning more than an order of magnitude (S=5–70 mm/m) with hydraulic forcing scaled relative to slope-specific entrainment thresholds. Across all slopes, sediment flux decreased with increasing conditioning time, indicating progressive riverbed strengthening. Strengthening rates increased strongly with slope, reflecting more effective stabilization by low flows. At steep slopes, reduced hydrodynamic lift forces may combine with ongoing granular rearrangement during low flows to progressively stabilize the bed, increasing grain resistance to entrainment. These results demonstrate that slope exerts a strong control on riverbed recovery following floods. Because strengthening rates scale predictably with channel gradient, slope may provide a first-order parameter for predicting flow-history-driven evolution of entrainment thresholds in sediment transport models, with implications for erosion rates, channel form, and the distribution of geomorphic work across watersheds.