Channel morphology transition between colluvial-headwater channels and concave-fluvial channels co-locates with spatial patterns of historic debris flow runouts, in-channel bedrock strength, and in-channel sediment storage

Abstract:

Steep headwater-colluvial channels comprise a large fraction of mountain channel networks, control variations in sediment flux reaching larger rivers, and are the initiation site of many destructive debris flows. Higher stream gradients and less-concave longitudinal profiles distinguish headwater channels from more-concave channels with lower gradients downstream, and this contrast in channel morphology is commonly attributed to a process transition where headwater channels are primarily carved by debris flows. Few observational data are available to test this hypothesis. Here we present Schmidt hammer measurements of in-channel bedrock and lidar-derived measurements of valley width from the San Gabriel Mountains, CA (SGM) and North San Jacinto Mountains, CA (NSJM). These proxies of intact rock strength and sediment fill volume reflect the time since weathered bedrock was stripped from the channel bed and sediment fill was cleared, and we use our measurements to delineate between the activity of spatially intermittent debris flows and more evenly-distributed fluvial processes. Schmidt hammer rebound values increase with increasing drainage area, suggesting erosion by discrete debris flows, assuming the recurrence interval between bedrock-scouring events decreases downstream as flows from different channel heads are routed through trunk streams. In the NSJM, cross-valley sediment fill width increases downstream and peaks at drainage areas corresponding to the base of headwater channels. Sediment grain size is also largest at the base of headwater channels in the SGM and NSJM. These findings suggest that many debris flows deposit material at the base of headwater channels, which matches observations from debris flow events that occurred in February 2019 in the NSJM and between 2014-2015 in the SGM, where 60-80% of flows traversed headwater channel domains, but only ~20-30% of flows reached lower-gradient channels and fans. The morphologic transition from headwater to concave channels occurs at different drainage areas in the NSJM (0.5-2km²) and SGM (0.08-1 km²), but this drainage-area offset is mirrored by downstream changes in sediment grain size, cross-valley sediment fill, and Schmidt hammer rebound value, reinforcing links between debris flow activity and headwater channel morphology.