Abstract. To explore the sensitivity of rivers to blocking from landslide
debris, we exploit two similar geomorphic settings in California's
Franciscan mélange where slow-moving landslides, often referred to as
earthflows, impinge on river channels with drainage areas that differ by a
factor of 30. Analysis of valley widths and river long profiles over
∼19 km of Alameda Creek (185 km2 drainage area) and
Arroyo Hondo (200 km2 drainage area) in central California shows a
very consistent picture in which earthflows that intersect these channels
force tens of meters of gravel aggradation for kilometers upstream, leading
to 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 upstream
of locations where earthflows impinge on its channel. Hydraulic and
hydrologic data from United States Geological Survey (USGS) gages on Arroyo Hondo and the Eel River, combined
with measured size distributions of boulders input by landslides for both
locations, suggest that landslide derived boulders are not mobile at either
site during the largest floods (>2-year recurrence) with field-measured flow depths. We therefore argue that boulder transport capacity is
an unlikely explanation for the observed difference in sensitivity to
landslide inputs. At the same time, we find that earthflow fluxes per unit
channel width are nearly identical for Oak Ridge earthflow on Arroyo Hondo,
where evidence for blocking is clear, and for the Boulder Creek earthflow on
the Eel River, where evidence for blocking is absent. These observations
suggest that boulder supply is also an unlikely explanation for the observed
morphological differences along the two rivers. Instead, we argue that the
dramatically different sensitivity of the two locations to landslide
blocking is related to differences in channel width relative to typical
seasonal displacements of earthflows. A synthesis of seasonal earthflow
displacements in the Franciscan mélange shows that the channel width of
the Eel River is ∼5 times larger than the largest annual
seasonal displacement. In contrast, during wet winters, earthflows are
capable of crossing the entire channel width of Arroyo Hondo and Alameda
Creek. In support of this interpretation, satellite imagery shows that
immobile earthflow-derived boulders are generally confined to the edges of
the channel on the Eel River. By contrast, immobile earthflow-derived
boulders 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 upstream
propagation of base-level signals, valley-blocking earthflows may therefore
promote the formation of so-called “relict topography”.
River channel width controls blocking by slow-moving landslides in California's Franciscan mélange