Sediment controls on the transition from debris flow to fluvial channels in steep mountain ranges

Steep channel networks commonly show a transition from constant-gradient colluvial channels associated with debris flow activity and concave-fluvial channels downstream. The trade-off between debris flow and fluvial erosion in steep channels remains unclear, which obscures connections among topography, tectonics, and climate in steep landscapes. We analyzed steep debris-flow-prone channels across the western US and observe: 1) similar maximum channel gradients across a range of catchment erosion rates and geologic settings; and 2) lengthening colluvial channels with increases in sediment grain size. The correspondence between sediment grain size and colluvial channel length led us to test a hypothesis that steep channel gradients are controlled by thresholds of motion for mass-wasting failure of channel bed sediment and thresholds of sediment motion by fluvial entrainment. We calculated discharges needed to mobilize sediment by both mechanisms across channel networks in the San Gabriel Mountains (SGM) and northern San Jacinto Mountains in southern California (NSJM). Across steep colluvial channels in both landscapes, sediment is more likely to be mobilized by mass-wasting processes, which are less-sensitive to sediment grain size, in agreement with observations from imagery bracketing storms. Discharges with decadal recurrences are below fluvial entrainment thresholds but above mass-wasting entrainment thresholds for D50 (median) sediment sizes in colluvial channels. In lower-gradient concave channels downstream, discharges in decadal storms exceed fluvial entrainment thresholds but are lower than mass-wasting entrainment thresholds for D50-sized sediment. In both landscapes, fluvial channels progressively steepen downstream compared to gradients needed to mobilize sediment cover, which we interpret to reflect downstream increases in sediment flux. Coarser sediment supply in the NSJM than the SGM increases threshold discharges needed to mobilize sediment by fluvial entrainment, which increases total channel relief in the NSJM by (1) extending colluvial channels shaped by debris flows and (2) increasing fluvial channel gradients. Together, our analyses show how differing sensitivity of fluvial and debris flow processes to sediment grain size impacts the partitioning of colluvial and fluvial regimes in headwater channel networks.