Boulders modulate hillslope-channel coupling in the northern Alaska Range

Abstract

Active orogens balance tectonic rock uplift with erosion, commonly via river incision coupled to landslide denudation of “threshold” hillslopes, but sediment’s role in this feedback is unclear. We report fluvial geometry, and sediment size, prevalence, and mobility across two ≤600-m-tall gneissic northern Alaska Range anticlines that sustain steep landslide-clad hillslopes but differ 10× in late Pleistocene−recent rock uplift rate. Enigmatically, the river steepens and narrows prominently across the fold experiencing slow surface uplift (∼0.5 mm/yr) but remains low-gradient and wide downstream across the anticline undergoing rapid differential rock uplift (∼5 mm/yr). Frequent bedload mobilization across both folds implies fluvial equilibration to sediment transport despite discrepant channel forms and similarly prevalent hillslope-derived boulders. Boulder prevalence correlates significantly with channel slope and width on the slowly uplifting anticline, but weakly on the rapidly uplifting anticline. Strong correlations across the tectonically quiescent anticline may reflect local incision-suppressing boulder aggradation that forces the channel to steepen and narrow, consistent with field observations. Conversely, weak correlations across the rapidly uplifting anticline imply that boulders may modulate expected tectonic channel adjustment by preferentially aggrading to subdue slope, and deflecting frequently mobile bedload to drive lateral erosion that maintains channel width, steepens adjacent hillslopes, and perpetuates hillslope-channel coupling. Hence, hillslope-derived boulders may occupy important roles in regulating feedbacks between river incision and landslide erosion that differ fundamentally at high and low tectonic rates.