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Abstract. To explore the sensitivity of rivers to blocking from landslidedebris, we exploit two similar geomorphic settings in California'sFranciscan mélange where slow-moving landslides, often referred to asearthflows, impinge on river channels with drainage areas that differ by afactor of 30. Analysis of valley widths and river long profiles over∼19 km of Alameda Creek (185 km2 drainage area) andArroyo Hondo (200 km2 drainage area) in central California shows avery consistent picture in which earthflows that intersect these channelsforce tens of meters of gravel aggradation for kilometers upstream, leadingto apparently long-lived sediment storage and channel burial at these sites.In contrast, over a ∼30 km section of the Eel River (5547 km2 drainage area), there are no knickpoints or aggradation upstreamof locations where earthflows impinge on its channel. Hydraulic andhydrologic data from United States Geological Survey (USGS) gages on Arroyo Hondo and the Eel River, combinedwith measured size distributions of boulders input by landslides for bothlocations, suggest that landslide derived boulders are not mobile at eithersite during the largest floods (>2-year recurrence) with field-measured flow depths. We therefore argue that boulder transport capacity isan unlikely explanation for the observed difference in sensitivity tolandslide inputs. At the same time, we find that earthflow fluxes per unitchannel width are nearly identical for Oak Ridge earthflow on Arroyo Hondo,where evidence for blocking is clear, and for the Boulder Creek earthflow onthe Eel River, where evidence for blocking is absent. These observationssuggest that boulder supply is also an unlikely explanation for the observedmorphological differences along the two rivers. Instead, we argue that thedramatically different sensitivity of the two locations to landslideblocking is related to differences in channel width relative to typicalseasonal displacements of earthflows. A synthesis of seasonal earthflowdisplacements in the Franciscan mélange shows that the channel width ofthe Eel River is ∼5 times larger than the largest annualseasonal displacement. In contrast, during wet winters, earthflows arecapable of crossing the entire channel width of Arroyo Hondo and AlamedaCreek. In support of this interpretation, satellite imagery shows thatimmobile earthflow-derived boulders are generally confined to the edges ofthe channel on the Eel River. By contrast, immobile earthflow-derivedboulders jam the entire channel on Arroyo Hondo. Our results imply that lower drainage area reaches of earthflow-dominated catchments may be particularly prone to blocking. By inhibiting the upstreampropagation of base-level signals, valley-blocking earthflows may thereforepromote the formation of so-called “relict topography”.