Slope-dependent riverbed strengthening and the evolution of sediment mobility in gravel-bed rivers

The threshold for motion exerts a fundamental control on sediment transport, channel morphology, and 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 slope modulates riverbed strengthening during low flow periods. We conducted flume experiments across slopes spanning over an order of magnitude (S = 0.005−0.07 m/m) with hydraulic forcing scaled relative to slope-specific entrainment thresholds. Across all slopes, sediment flux decreased with increasing low-flow duration, indicating progressive riverbed strengthening. Strengthening rates increased with slope, reflecting more effective stabilization by low flows. At steep slopes, we hypothesize that reduced hydrodynamic lift forces combine with granular rearrangement during low flow to progressively stabilize the riverbed. 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.