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Fluvial boulder transport controls valley blocking by earthflows in the California Coast Range, USA

Abstract. At two similar sites in California's Franciscan Mélange (Arroyo Hondo, central California and the Eel River, northern California), earthflows impinge on river channels with drainage areas that differ by a factor of 30. We compare these sites to explore how river flow depth, width, and velocity control river resilience to valley blocking and aggradation that occurs as earthflows deliver large quantities of coarse sediment to the channel network. We measure the size distribution of earthflow-derived boulders (> 30 cm) delivered to each channel and use USGS stream gages to quantify the mobile fraction of boulders for a 2-year recurrence interval flood. For Arroyo Hondo, only the top ~ 10 % of boulders are immobile (> 2.4 m diameter), however, this portion of the distribution represents ~ 80 % of the volume of coarse, earthflow-derived material. For the Eel River, the top ~ 1 % of boulders are immobile (> 4.9 m diameter), a fraction that represents only ~ 20 % by volume. Satellite imagery shows that immobile boulders in Arroyo Hondo jam the entire channel and coincide with knickpoints and aggradation for km’s upstream. By comparison, immobile boulders in the Eel River are sparsely distributed and confined to the edges of the channel. Moreover, the Eel River valley and long profile show little evidence of perturbation despite numerous well documented active earthflows along its length. This contrast suggests valley blocking is very sensitive to the mobility of the coarsest fraction of sediment. In this way, earthflow-impacted channels may act like step-pool channels, where channel-spanning boulder jams locally impede coarse sediment transport. In support of this view, the ratio of channel width to the threshold diameter for immobile boulders on Arroyo Hondo is ~ 5 (as opposed to ~ 25 for the Eel River), suggesting the river is susceptible to jamming. Our results imply that the lower drainage area, upper portions of earthflow-dominated catchments may be particularly prone to blocking. Valley blocking, in turn, may inhibit propagation of base-level signals and promote formation of relict topography and fluvial hanging valleys.
