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”.
Introduction
River incision into bedrock drives landscape change in unglaciated settings
and is the key process linking tectonics and topography (Whipple, 2004). However, the process of river
incision is sensitive to the amount and caliber of coarse sediment supplied
from hillslopes (Sklar and Dietrich, 2001, 2004)
such that coarse sediment input can accelerate (Cook et al.,
2013) or arrest incision (Bull, 1990). This
non-linearity reflects the dual role of coarse sediment in providing
abrasive tools and inhibiting incision when deposited on the bed (Gilbert, 1877).
The dependence of river incision on hillslope-derived coarse sediment input
also means that river incision, which provides the lower boundary condition
for hillslopes, and landsliding, which delivers coarse sediment to channels,
are coupled (Golly et al.,
2017; Ouimet et al., 2007; Schuerch et al., 2006). The apparent strength of
this coupling, however, varies widely. On the one hand, river aggradation
and valley blockages triggered by rockfalls and rock avalanches can persist
for timescales as long as 104 years (Korup et al., 2006). Similarly,
elevated coarse sediment loads following co-seismic landslides can attenuate
over timescales as long as centuries (Stolle et al., 2019; Yanites et al.,
2010). These examples suggest that landsliding provides a strong negative
feedback on river incision by causing long-lived burial events and hence
hiatuses in downcutting (Miller et
al., 2016; Ouimet et al., 2007; Yanites et al., 2010). These examples also
suggest that valley aggradation or incision following a landslide may occur
long after evidence for the landslide itself is recognizable in the
landscape. On the other hand, in some settings, little evidence of valley
blocking is seen despite extremely high rates of landsliding (Korup et al., 2010), suggesting a contrasting view
in which rivers are not strongly perturbed following landslides and in which
landsliding occurs essentially passively in response to river incision (Burbank et al., 1996; Larsen and
Montgomery, 2012).
Work devoted to the problem of landslide dam formation from the perspective
of landslide processes emphasizes both the rates of material delivery to
channels as well as the width of river valleys into which debris is
delivered as key controls on dam formation (Costa and Schuster, 1988; Korup,
2002). That said, comparatively less work has focused on fluvial controls on
the resilience of rivers to landslide inputs. With this in mind, here we
explore what governs river resilience to valley blocking by landslide
debris. Towards this end, we exploit two similar geomorphic settings in the
California Coast Range where slow-moving landslides, often referred to as
earthflows, impinge on river channels with drainage areas that differ by a
factor of 30. By comparing mapped landslide locations to long profile and
valley morphology, we establish locations where valley blocking from
earthflows has occurred at each site. We then explore factors that govern
apparent contrasts in river resilience to valley blocking in otherwise
similar geomorphic and geologic settings.
In this paper, we consciously use the term valley blocking instead of
damming. While valley blocking from earthflows, like damming, can cause
aggradation for kilometers upstream to depths of several tens of meters, it
is unusual that earthflows deposit sediment rapidly enough to cause the
formation of lakes. Hence, to avoid confusion, here we adopt the term valley
blocking.
Geologic and geomorphic setting
The Franciscan Complex is an assemblage of variably deformed and
metamorphosed rock units formed in a subduction zone during the Mesozoic and
early Cenozoic eras (Wakabayashi, 1992). With
widespread occurrence throughout the state of California (Fig. 1),
Franciscan lithologies include primarily detrital sedimentary rocks such as
sandstones and argillaceous mélanges that are well known for their low
strength and high susceptibility to slope failure. Many documented instances
of earthflows, in particular, in California occur within these units (Iverson
and Major, 1987; Keefer and Johnson, 1983; Kelsey, 1978; Roering et al.,
2015; Scheingross et al., 2013). Earthflows are characterized by a flow-like
appearance and persistent motion over decades to centuries (Hungr et al., 2014). They form above fine-grained
bedrock in plastic, clayey soil and are commonly large (> 500 m
long) and deep (> 5 m). They typically move at rates less than
∼10 m yr-1 (Baum et al., 2003); however,
they can also display more rapid but short-lived surge-like events
approaching ∼ m d-1 (Hungr et al., 2014). Active earthflows
frequently extend from ridge tops to valley bottoms (Mackey
and Roering, 2011) and are classically described as having an “hourglass”
planform outline, with a bowl-shaped source area, an elongated transport
zone and a lobate toe (Keefer and Johnson, 1983).
Notwithstanding their name and appearance, most earthflow movement occurs by
sliding along discrete basal and lateral shear surfaces (Fleming
and Johnson, 1989; Keefer and Johnson, 1983; Simoni et al., 2013; Vulliet
and Hutter, 1988; Zhang et al., 1991). In this paper, we exploit two
locations in the California Coast Range where earthflows impinge on
channels of greatly differing scale. Both of these locations are underlain
by Franciscan Complex lithologic units. Below, we describe each location
separately.
Overview of the study region. Franciscan Complex rocks are shown
in purple. Red boxes indicate the locations of the two field locales.
Arroyo Hondo and Alameda Creek
Arroyo Hondo (200 km2) and upper Alameda Creek (185 km2) drain a
rugged region of the northern Diablo Range, northeast of San Jose,
California (Figs. 1 and 2). Their confluence occurs just downstream of
Calaveras Dam, which impounds Calaveras Reservoir, the largest reservoir in
the San Francisco Bay Area. Where each creek crosses the actively uplifting
Diablo Range, it has incised a deep (∼600 m) canyon into
Franciscan formation sandstone and mudstone mélange. The walls of these
canyons are draped with earthflows (Figs. 2 and 3). One of them, Oak
Ridge earthflow, was studied by Nereson and Finnegan (2019), who analyzed its historical motion from aerial photos that span
1937–2017. Within the persistently active ∼100 m wide
transport zone of the earthflow, the mean velocity was ∼ 2.15 m yr-1 over this time interval (Nereson and Finnegan, 2019). We
use field observations and detailed measurements of boulder size
distributions at Oak Ridge earthflow as a reference site from which we make
more generalized inferences about the relationship between earthflows and
valley blocking in Alameda Creek and Arroyo Hondo. Here, we analyze
approximately 11 km of Alameda Creek and 8 km of Arroyo Hondo. These are
reaches where the authors have performed extensive field reconnaissance. A
United States Geological Survey (USGS) gage on Arroyo Hondo is located within the study section considered
here (https://waterdata.usgs.gov/usa/nwis/uv?11173200, last access: 23 July 2018) (Fig. 2). At this location, Arroyo Hondo has a drainage area of 200 km2.
Annual rainfall at Oak Ridge earthflow is 53 cm, most of which occurs
between October and May (Nereson and Finnegan, 2019).
Shaded relief map of the study reaches of the Alameda Creek and
Arroyo Hondo watersheds. White boxes highlight large mapped earthflows in
the study area, which are shown in more detail in Fig. 3. Rivers channels
are indicated with blue lines, except where they intersect active earthflows
(red lines). Areas of light red shading show regions where the InSAR
analysis indicated line-of-site velocities in excess of 3 cm yr-1.
Shaded relief maps of three large earthflows on Alameda Creek (a–c) and two on Arroyo Hondo (d–e). Dashed white lines indicate the mapped
edge of earthflow toes, and red lines indicate where river channels (shown
in blue) intersect earthflows. Areas of red shading show regions where the
InSAR analysis indicated line-of-site velocities in excess of 3 cm yr-1.
Eel River
We also exploit a study site developed by Mackey and Roering (2011) along a
∼30 km section of the main stem Eel River between Dos Rios
and Alderpoint (Figs. 1 and 4) in the northern California Coast Range in
Mendocino, Humboldt and Trinity counties. At this location, the Eel River
cuts an ∼800 m deep canyon into actively uplifting rocks of
the Central Belt of the Franciscan Complex (McLaughlin et
al., 2000). The Central Belt consists of mudstone mélange, similar to
Arroyo Hondo and Alameda Creek, that surrounds coherent blocks of various
lithologies that can be as large as entire mountains (Roering et al.,
2015). At this location, Mackey and Roering (2011) tracked the historical
motion of 122 earthflows from 1944 to 2006. Over this period, the median
annual sliding velocity of all landslides was 0.4 m yr-1. More recent work (Bennett et al., 2016a) revealed a significant
deceleration of these earthflows during the historic California drought from
2012–2015, and both recent acceleration and activation of new slides during
the extremely wet winter of 2016–2017 (Handwerger
et al., 2019a, b). We analyze river data from a USGS gage located
at Fort Seward (https://waterdata.usgs.gov/ca/nwis/uv?site_no=11475000, last access: 23 July 2018), approximately 12 km downstream of Alderpoint. At this location,
the Eel River has a drainage area of 5547 km2. Because there are no
major tributary junctions between the study reach and the Fort Seward gage,
we assume the Fort Seward gage is approximately representative of the
conditions within the study section. Annual rainfall at Alderpoint is 130 cm
(Mackey and Roering, 2011). We use a reference site at an active earthflow,
referred to as the “Mile 201” slide by Mackey and Roering (2011), just
downstream from the confluence of Kekawaka Creek with the Eel River (Fig. 4). At this location, we make detailed measurements of boulder size
distributions being supplied to the Eel River by earthflows via high-resolution aerial imagery.
Shaded relief map of the Eel River site. The thalweg of the Eel
River is shown in blue. Areas of red shading are active earthflows mapped by
Mackey and Roering (2011). River flow is from the lower right to upper left.
MethodsLandslide impacts on channels
We use river long profile morphology (along with floodplain width, described
below) to assess the fluvial response to earthflow inputs. Earthflow
deposits are commonly comprised of large boulders, which can lead to steep boulder
cascades downstream of landslide blockages and in turn drive fluvial
aggradation upstream (Kelsey, 1978). Thus, the topographic
signature of valley-blocking landslides in river long profiles is an
anomalously low-gradient reach, representing the upstream aggradational
section, that grades downstream to the lip of a steep knickpoint, which
represents the valley-blocking deposit (Ouimet et al.,
2007). To highlight such reaches, we linearly detrend the river long
profiles for each of the three reaches examined. The residual topography
following detrending highlights locations where the channel departs from a
smoothly graded profile. In addition, the amplitude of the residual
topography provides a direct measurement of the height of valley blockages
and hence the depth of fluvial aggradation upstream of blockages.
Valley aggradation in a steep-walled canyon leads naturally to floodplain
widening simply by virtue of the triangular cross-section of a valley
(Mey et al., 2015; Reneau and
Dietrich, 1991). For this reason, we also make measurements of local
floodplain width to complement our channel slope measurements. The logic
here is that deep aggradation upstream of landslide blockages should be
reflected in local floodplain width. We use lidar-derived slope maps to help
identify the slope break that marks the intersection of the steep canyon
wall with the low-gradient valley bottom alluvial deposits. We hand
digitized a line corresponding to this slope break along each side of the
three canyon reaches examined here. We then rasterized this line and used
ArcGIS to calculate the Euclidean distance from both the right and left
edges of the floodplain. The sum of these Euclidean distance maps within the
active floodplain yields an approximation of the local floodplain width,
which we extract at each point where we measure elevation. To highlight
potentially landslide-impacted river reaches, we look for points that mark
rapid changes in valley width (from wide to narrow) and rapid changes in
river slope (from low to high).
For both sites, we take advantage of lidar-derived topography data to make
measurements of river channel morphology. For the Eel River site, we use lidar
data as described by Mackey and Roering (2011). For the Alameda Creek/Arroyo
Hondo site, we use one-ninth second USGS NED data. We hand digitized thalweg
profiles for both study locations using shaded relief maps. We extracted
elevation points every 100 m for the Eel River and every 10 m for the Arroyo
Hondo and Alameda Creek sites. This difference in spacing reflects the
approximate difference in channel width for the two locations.
Quantification of boulder size distributions
We use Google Earth imagery, which we exported to ArcGIS after
re-georeferencing, to map boulder size distributions entering channels at
the toes of active earthflows at our two reference sites. The toe of Oak
Ridge earthflow is currently collapsing along a series of rotational
failures into Arroyo Hondo (Nereson and Finnegan, 2019), which,
in combination with prehistoric motion of the earthflow toe, has resulted
in an accumulation of large, unsorted earthflow-derived boulders in the
channel of Arroyo Hondo at the base of the earthflow. We digitized all
visible boulders (n=329) as ellipses, which we then fit with rectangles
to quantify the major and minor axis lengths of the boulders. The imagery
enables us to identify boulders down to 30 cm in diameter. Because of the
proximity of boulders to the earthflow toe and the absence of sorting in the
field, we treat the distribution of boulders at the toe of Oak Ridge
earthflow as representative of the coarse fraction of material (larger than
the 30 cm detection limit) that is eroding out of the earthflow once its
fine matrix has been winnowed away (e.g., Kelsey, 1978). This interpretation
is supported by the fact that it is impossible to differentiate individual
grains on bar surfaces upstream of the earthflow toe in aerial imagery. In
other words, the bulk of the distribution of bedload that is moved by the
river appears to fall below the detection limit in the aerial imagery.
For the Eel River reference site, we divided the channel into three domains
where we digitized boulders separately. Along the north (river right) bank
of the Eel River at the toe of the Mile 201 slide is an accumulation of unsorted
boulders similar to what is observed at Arroyo Hondo along the toe of Oak
Ridge. However, in contrast to the Arroyo Hondo reference site, within the
thalweg of the Eel River, large boulders are absent and weak sorting is
apparent. Hence, we treat the population of boulders along the north bank (n=413) as representative of the coarse fraction (>30 cm) that
is eroding out of the mélange that comprises the body of the Mile 201
earthflow, whereas we consider the thalweg to be more influenced by fluvial
transport. The south (river left) bank of the Eel River at this site is
similar to the north bank. However, we treat the distribution of boulders
from this site (n=706) separately because we are unsure whether this
material is sourced from the Mile 201 slide or the active earthflow that
enters this same location from the south (Mackey and Roering, 2011). Like in
Arroyo Hondo, it is impossible to differentiate individual grains on bar
surfaces away from earthflow toes in aerial imagery. Hence, again, we assume
that most of the distribution of bedload that is moved by the river appears
to fall below the detection limit in the imagery.
Hydrology
The USGS gage on Arroyo Hondo is located within the study reach, roughly 2 km downstream of Oak Ridge earthflow. The gage record includes 36 years of
annual peak flood measurements and 248 field measurements of discharge,
width and cross-sectional area during both high- and low-flow events. The
USGS gage on the Eel River is located approximately 25 km downstream of the
Highway 201 slide and roughly 12 km downstream of the edge of the lidar data
considered here. The gage record includes 62 years of annual peak flood
measurements and 364 field measurements of discharge, width and
cross-sectional area during both high- and low-flow events.
We calculated the recurrence period associated with the annual peak flood
measurements for each gage according to standard methods (Dunne
and Leopold, 1978). We then estimated the magnitude of the 2-year recurrence
interval flood from the record. To accomplish this, we located the
recurrence intervals that bracketed 2 years in the record and fit a line
between these two points. Finally, using linear interpolation, we determined
the approximate magnitude of the 2-year recurrence interval flood, which we
use as a representative high-flow event in our analysis. In alluvial
channels, the 2-year recurrence interval flood often corresponds to
“bankfull” flow (Wolman and Miller,
1960). For this reason, the 2-year flood is commonly interpreted as the
“formative” flow with respect to bankfull hydraulic geometry. Here, we use
the 2-year flood, however, simply as a representative flood event that would
mobilize the bed in most self-formed alluvial channels but not necessarily
in a channel overwhelmed with landslide debris.
Fortuitously, field measurements of discharge and hydraulic geometry for
both gages bracket the 2-year recurrence interval flood. We divide measured
cross-sectional area by width to quantify mean flow depth by assuming a
rectangular geometry for each gage. We then plotted mean flow depth versus
discharge for each record. The relationship between discharge and flow
depth was well fit with a power–law relationship for the Eel River record.
Hence, for this record, we simply use the best-fitting power–law relationship
to find the mean depth associated with the 2-year recurrence interval
flood, as well as for the maximum flood discharge during which field
measurements were made. For Arroyo Hondo, a power law does not fit the
relationship between discharge and flow depth at the discharge near the
2-year recurrence interval flood. Consequently, for this record, we used a
linear fit for discharge greater than 20 m3 s-1, which fits the data
well in this region. We then apply this linear fit in order to determine the
mean flow depth associated with the 2-year recurrence interval flood on
Arroyo Hondo, as well as for the maximum flood discharge during which field
measurements were made.
Landslide identification
For the Eel River site, we use the landslide mapping of Mackey and Roering (2011), who identified 122 active individual earthflows within the Eel River
study site using historical aerial photos (Fig. 4). More recent studies
using optical images as well as satellite and airborne radar interferometry show that the majority of
these landslides are still active (Bennett
et al., 2016a, b; Handwerger et al., 2015, 2019b)
For Arroyo Hondo and Alameda Creek, we used a combination of airborne
synthetic aperture radar interferometry (InSAR), lidar topography and field
reconnaissance to identify slow landslides that are either currently active
or have been active in the recent geomorphic past.
Based on our own field reconnaissance in the area, as well as through
interpretation of lidar-derived topographic maps, we have identified several
large earthflows within the field area that clearly impinge on the channels
of Arroyo Hondo and Alameda Creek (Figs. 2 and 3). We also process radar
interferometry data from the NASA/JPL airborne uninhabited aerial vehicle synthetic
aperture radar (UAVSAR) platform to identify active landslides between 2009
and 2017 (Appendix A). UAVSAR operates with a L-band radar wavelength
(∼24 cm) and collects data at this location approximately two times per year along track 23503 (aircraft moving at 230∘ and
looking at 140∘). We processed 31 interferograms using the InSAR
Scientific Computing Environment (ISCE) software package developed at
JPL/Caltech and Stanford (Rosen et al., 2012). We remove topographic contributions to the phase
using a 12 m digital elevation model (DEM) from the DLR TanDEM-X satellites. We also reduce InSAR
phase noise using a standard power spectral filter with a value of 0.5 (Goldstein and Werner, 1998). Finally, we selected seven high-quality interferograms (i.e., minimal unwrapping errors, high coherence) to
compute an average line-of-sight (LOS) velocity map for landslides within the
study area.
ResultsLong profile and valley width
Figures 5a, 6a and 7a show elevation long profiles of Arroyo Hondo, Alameda
Creek and the Eel River, respectively. Also indicated on the figures are
the locations where landslides intersect river channels, as shown in Figs. 2–4. For the Eel River, landslide locations come from Mackey and Roering (2011). For Alameda Creek, landslide locations are, again, based on a
combination of field reconnaissance, lidar interpretation and InSAR.
(a) Elevation long profile of Arroyo Hondo. Locations where the channel intersects mapped earthflows in Figs. 2 and 3 are shown in red and labeled. (b) Detrended elevation profile of Arroyo Hondo for the reach shown in panel (a). (c) Valley bottom width for Arroyo Hondo normalized by the mean valley bottom width over the reach shown in panel (a). In panels (a)–(c), the red boxes highlight earthflow-impacted reaches.
(a) Elevation long profile of Alameda Creek. Locations where the channel intersects mapped earthflows in Figs. 2 and 3 are shown in red and labeled. (b) Detrended elevation profile of Alameda Creek for the reach shown in panel (a). (c) Valley bottom width for Alameda Creek normalized by the mean valley bottom width over the reach shown in panel (a). In panels (a)–(c),
the red boxes highlight earthflow-impacted reaches.
(a) Elevation long profile of the Eel River. Locations where the channel intersects mapped earthflows in Fig. 4 are shown in red. (b) Detrended elevation profile of the Eel River for the reach shown in panel (a). (c) Valley bottom width on the Eel River normalized by the mean valley bottom width over the reach shown in panel (a). Red rectangles highlight earthflow-impacted reaches in panels (a)–(c). BC indicates location of Boulder Creek
earthflow.
Figures 5b–c and 6b–c show detrended long profiles and valley width
(normalized to the mean for each river), respectively, for Alameda Creek and
Arroyo Hondo. For both channels, mapped landslide locations coincide with
locations along the river marked by rapid changes in valley width from wide
upstream to narrow downstream and large steps in the elevation long profile
that separate anomalously low-gradient reaches upstream from anomalously
steep reaches downstream. We also note that the sharpest river slope
increase associated with earthflows along Alameda Creek and Arroyo Hondo
occurs at Oak Ridge earthflow, which is the only failure in the region that
is both currently active and coupled to a river channel (Arroyo Hondo)
according to InSAR (Figs. 2 and 3) and feature tracking (Nereson and Finnegan, 2019).
In contrast, Fig. 7b–c show that active landslides along the Eel River
have no obvious impacts on long profile morphology and valley widths. At no
location along the Eel River section is the amplitude of the detrended river
elevation profile larger than the 2-year flow depth, suggesting that most of
the residual elevation is related to gravel bars, which are abundant along
the reach in question. In addition, except for one spot along the Eel River
section, valley bottom width is always less than a factor of 2 greater
than the mean.
Boulder size distributions
Figure 8 shows empirical cumulative distribution functions (CDFs) of the
minor axis of boulders mapped at the toe of Oak Ridge earthflow, and for the
right and left banks of the Eel River at the toe of the Mile 201 slide.
Although the distributions diverge below ∼1 m, the curves are
quite similar for boulders larger than 1 m. A two-sample Kolmogorov–Smirnov
test for the portions of each of the three measured boulder distributions
above 1 m is unable to reject the hypothesis (at the 5 % significance
level) that the three boulder populations are drawn from the same
distribution. For the Eel River, the median measured boulder size is 1.02 m
and the 84th percentile is 1.90 m. For Arroyo Hondo, the median
measured boulder size is 0.82 m and the 84th percentile is 1.83 m.
Empirical cumulative distribution functions for the minor
axes of earthflow-derived boulders at the toe of Oak Ridge earthflow and for
the banks of the Eel River site at the Mile 201 slide. The vertical lines
indicate the threshold mobile grain diameter for the largest field-measured
flow depths at each site (see Figs. 9 and 10).
Hydrology
Figures 9 and 10 show calculated mean flow depths based on field
measurements of channel width and cross-sectional area for each gage. As noted
earlier, the 2-year recurrence interval event (shown in red in each
figure) falls within the range of field measurements for the two sites,
making accurate interpolation of the characteristic 2-year recurrence
interval depth straightforward. Table 1 reports both the 2-year and largest
measured mean flood depths for each gage.
Mean flow depth versus measured discharge for the USGS Arroyo
Hondo gage. Red square indicates the interpolated 2-year recurrence
interval depth.
Mean flow depth versus measured discharge for the USGS Eel River
gage at Fort Seward. Red square indicates the interpolated 2-year
recurrence interval depth.
Channel characteristics for the two reference locations.
We rearrange the Shields equation to solve for the threshold gravel size
that can be moved assuming all of the shear stress acting on the channel bed
is available to transport sediment:
D=τρs-ρgτc∗.
In Eq. (1), D represents gravel diameter, τ is the mean channel bed
shear stress, ρ and ρs are the density of water and
sediment, respectively, g is the acceleration due to gravity, and τc∗ is the critical Shields stress. τ is calculated via
the depth slope product, ρghs, where h is mean flow depth and s is channel
slope. For the Arroyo Hondo USGS gage, the threshold gravel diameter
associated with a 2-year flood is 0.31 m. For the largest recorded flood
at the gage, the threshold gravel diameter is 0.51 m (Fig. 8). For the Eel
River gage, the threshold gravel diameter associated with a 2-year flood
is 0.22 m. For the largest recorded flood at the gage, the threshold gravel
diameter is 0.32 m (Fig. 8).
We note that these estimates ignore the possible morphodynamic feedbacks
that might result from the deposition of large boulders in a channel. On
the one hand, landslide derived boulder deposits are steep relative to
points upstream and downstream (Figs. 5a and 6b), suggesting that the
deposition of landslide debris might lead to conditions more favorable to
coarse sediment transport. On the other hand, large boulders exert
substantial drag on the flow, which can completely offset increases in
coarse sediment transport capacity due to the steeper slopes of boulder
cascades (Schneider et al., 2016).
For this reason, we simply consider the coarse sediment transport capacity
of the river at the gage sites as an index of the river's ability to move
coarse landslide-derived debris independent of changes in bed morphology
caused by that debris.
Discussion and conclusionsWhat controls valley blocking?
Analysis of valley widths and river long profiles in Alameda Creek and
Arroyo Hondo shows a very consistent picture in which landslides that
intersect the channel force tens of meters of gravel aggradation for
kilometers upstream, leading to apparently long-lived sediment storage and
channel burial at these sites (Figs. 5a–c and 6a–c). In contrast to
Arroyo Hondo and Alameda Creek, the Eel River does not display knickpoints
at or aggradation upstream of locations where earthflows impinge on the
channel, such as at the Mile 201 slide (Fig. 7a–c). Because of the similar
mass-wasting processes operating in the two study sites, we can identify no
obvious explanation based on landslide processes alone for the paucity of
evidence for valley blocking on the Eel River site. Indeed, all else being
equal, we would expect more evidence for valley blocking at the Eel River
site given the much greater density of active slow landslides there compared
to the Arroyo Hondo and Alameda Creek sites. Given this apparently strong
difference in the response of these two river systems to similar hillslope
forcing, below we explore several potential explanations based on fluvial
geomorphology for the apparent resilience of the Eel River site to valley
blocking by slow landslides.
Coarse sediment transport capacity
The results of our gravel mobility calculations (Table 1) show that boulders
delivered by earthflows should be immobile once delivered to river channels
at both study locales. Only the very left tail of the size distribution of
boulders approaches the threshold for gravel mobility in both channels
during even the largest flood (>2-year recurrence) for which
field data exist at each gage (Fig. 8). That said, boulders are relatively
more immobile for a given Shields stress compared to gravel (Prancevic and Lamb, 2015). Hence, it is entirely
plausible that the entire distribution of boulder sizes delivered by
earthflows is immobile once delivered to channels in both locations. This
interpretation is supported by the fact that gravel bars downstream of the
two reference earthflow sites do not contain boulders and typically do not
contain clasts that are even discernable above the ∼30 cm
resolution of the imagery. In addition, from examination of historical
aerial photography in Google Earth for both locations, we were unable to see
unambiguous evidence for mobile boulders even following very wet winters
with large recorded flood events.
Coarse sediment supply
Given the relatively short time period covered by the gage data used in the
mobility analysis, we acknowledge the possibility that large and infrequent
floods move boulders over very long timescales (Cook et al., 2018). With this possibility
in mind, here we consider landslide sediment supply relative to transport
capacity as a potential driver of the differences in apparent sensitivity to
landslide blocking observed between our two study sites. Because the Eel
River is ∼10 times wider than Arroyo Hondo, it has a much
larger coarse sediment transport capacity than Arroyo Hondo just by virtue
of its width. Hence, it is possible that the resilience of the Eel River to
earthflow blocking is a consequence of its larger width and hence volumetric
transport capacity. To test this, we used data from Mackey and Roering (2011) to calculate the volumetric sediment flux per unit river width of the
Boulder Creek earthflow, which is the largest earthflow on the Eel River
(Fig. 4). We compare this calculation to the volumetric flux per unit river
width associated with Oak Ridge earthflow. The latter is calculated using
the mean velocity of Oak Ridge earthflow (2.15 m yr-1) (Nereson and Finnegan,
2019) multiplied by the width of the earthflow (∼100 m) and
the depth (∼8 m), as reported from electrical resistivity
surveys of the slide (Murphy et al., 2018). Although
the Boulder Creek earthflow has an order of magnitude larger volumetric flux
(∼15000 m3 yr-1) than Oak Ridge earthflow
(∼1700 m3 yr-1), the Eel River has an order of magnitude
larger channel width (125 m) than Arroyo Hondo (12 m). Hence, earthflow
fluxes per unit channel width at the two sites are nearly identical:
∼140 m2 yr-1 for Arroyo Hondo and ∼130 m2 yr-1 for the Eel River. Assuming a similar concentration of boulders
within the mélange at both sites, which is reasonable based on the
surface distribution of boulders that is apparent at both sites, this
calculation suggests that boulder fluxes per unit channel width at the two
sites are also likely to be comparable. Despite this similarity, there is no
evidence of blocking in the long profile of the Eel River at the location of
the Boulder Creek slide (∼ km 18 in Fig. 7a), whereas the
channel of Arroyo Hondo is clearly blocked at Oak Ridge (Fig. 5). For this
reason, we also rule out boulder supply relative to transport capacity as a
likely driver of the observed morphological differences on the two rivers.
Channel width and seasonal earthflow displacement
An alternative explanation for the resilience of the Eel River to landslide
blocking may be related to differences in channel width at both sites
relative to typical seasonal displacements of earthflows. Figure 11 shows a
histogram of seasonal earthflow displacement in the Franciscan mélange
from Nereson and Finnegan (2019), who used a physically based model to
interpolate between 22 air photos of Oak Ridge earthflow, and Kelsey (1978),
who used stakes to directly measure displacement on six earthflows in the
Van Duzen River watershed, a tributary of the Eel River, in northern California between 1973 and 1976.
Notably, the distribution of displacements, which is well fit with an
exponential distribution with a mean annual displacement of ∼4 m, contains the channel width of Arroyo Hondo (Fig. 11). By contrast, the
channel width of the Eel River is ∼ 5 times larger than the
largest annual displacement in the distribution. Thus, during wet winters
with large displacements, earthflows are capable of surging across the
entire channel of Arroyo Hondo. In contrast, on the Eel River, large
earthflow surges can impinge on but cannot cross the active channel. 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 (Fig. 12) where at each bank an active earthflow
enters the Eel River. By contrast, immobile earthflow-derived boulders jam
the entire channel on Arroyo Hondo (Fig. 13). We interpret this striking
difference in channel morphology as an indication of the ability of
earthflows to cross and hence deposit boulders within the entire width of
Arroyo Hondo.
Normalized histogram of annual earthflow displacements in the
Franciscan mélange; data are from Kelsey (1978) and Nereson and Finnegan (2019). Black line shows exponential fit between normalized frequency and
displacement. Vertical dashed lines indicate the channel width of Arroyo
Hondo and the Eel River.
The contrasting long profile and channel morphology of the two reference
locations thus suggests that the formation of valley blockages by earthflows
is very sensitive to the width of river channels relative to the
characteristic displacement of earthflows. In this way, channels where
landslides can cross the channel may act like step-pool channels, where
channel-spanning, landslide-derived boulder jams locally impede coarse
sediment transport. In support of this view, the observations of Golly et
al. (2017) and Brummer and Montgomery (2006) directly
linked channel-spanning boulder jams to channel-spanning landslides in two
mountainous settings. According to flume experiments (Zimmermann et al., 2010), particle jams
become much more likely when the channel width is less than 6 times the
84th percentile of the grain size distribution. Not coincidentally,
bulk friction angle (and hence stability) of chains of clasts is larger for
smaller chains, with an inflection in the relationship between frictional
stability and number of clasts at around six grains (Booth
et al., 2014). Notably, the ratio of channel width to the 84th
percentile diameter of earthflow-derived boulders on Arroyo Hondo is
∼6, suggesting that Arroyo Hondo and Alameda Creek are in a
regime where they are much more susceptible to jamming once boulders are
deposited on their beds. This interpretation also agrees with Costa and
Schuster (1988), who found that valley width is a clear control on landslide
dam formation because it is easier to block a narrow valley than a wide
valley.
The fact that the Eel River does not show evidence for valley blocking
likely reflects that fact that the channel is always able to flow around the
toe of earthflows. In addition, the
ratio of channel width to the 84th percentile diameter of
earthflow-derived boulders on the Eel River is ∼65, which is
well beyond the threshold associated with jamming. That said, over their
lifetimes, earthflows can travel hundreds of meters (Mackey and Roering,
2011; Nereson and Finnegan, 2019), distances that are in excess of what
would be required to block the Eel River. Yet the Eel River shows no
evidence for blocking; why? The matrix of earthflows in the Franciscan
mélange is very fine grained, making it easily transported in suspension
by rivers (Kelsey, 1978; Mackey and Roering, 2011). We speculate that
short-lived advances of earthflows into the Eel River during wet winters are
countered by the ability of the river to remove the matrix of the earthflow
during subsequent years, leading to a relatively stable position of the
earthflow toe relative to the channel despite long-term displacements that
are sufficient to cross the channel. We emphasize, however, that this
interpretation is only applicable to the relatively slow-moving earthflows
that are plentiful in the Eel River basin. Catastrophic landslides, which
can occur outside of the Franciscan mélange along the Eel River, have
dammed the Eel River in at least one location during the Pleistocene, resulting in
the formation of a landslide-dammed lake (Mackey et al., 2011).
Implications for river incision and landscape evolution
The results of our analysis imply that the Eel River, and wide rivers like
it, should be able to sustain vertical incision despite active earthflows.
In other words, in settings where earthflow surges are small in comparison
to channel width, landsliding may not represent a strong negative feedback
on river incision. In contrast, in settings such as Arroyo Hondo and Alameda
Creek, where surges can cross the entire channel, landsliding should
represent a strong negative feedback on vertical river incision by
triggering channel-spanning boulder jams that force aggradation over large
sections of river upstream of active landslides (Figs. 5a–c and 6a–c).
Notably, this difference in sensitivity to landslide input is independent of
the examined rivers' capacities to actually mobilize coarse debris.
Thus, 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” (Clark et
al., 2006; Schoenbohm et al., 2004), where the upper portions of watersheds
are unable to incise at the same rate as the mainstream. Indeed, Bennett
et al. (2016a) noted that tributaries to the Eel River are choked with
coarse sediment that significantly impedes river incision into bedrock and
hypothesized that earthflow-dominated catchments are prone to a so-called
“landslide cover effect”, which prevents or delays the upstream
propagation of base-level signals, thereby leading to the formation of
relict topography. Similarly, Korup et al. (2010) argued that sediment
inputs from rockfalls and glaciers have suppressed river incision into the
margin of the Tibetan Plateau, aiding in its apparent longevity. We note
that Ouimet et al. (2007) and Shobe et
al. (2016) also demonstrated a landslide cover effect via numerical modeling
of bedrock river incision.
Our results are generally supportive of these perspectives and offer a
simple mechanism for the instability that triggers the incisional shutdown
of earthflow-dominated channels. Moreover, because we identify a clear width
(and thus drainage area) dependence on the susceptibility of channels to
jamming from earthflow-derived boulders, our results imply that tributary
junctions are likely to mark boundaries between relict topography and
actively incising canyons. In other words, landslide or debris-flow-derived
boulder jams in narrow channels provide an alternative explanation for the
phenomenon of fluvial hanging valleys, where tributary channels in steep
canyons are apparently unable to incise at the rate of trunk channel
incision (Crosby et
al., 2007; Wobus et al., 2006).
We note that morphodynamic models for river response to landslide inputs (Croissant et al., 2017;
Ouimet et al., 2007; Shobe et al., 2016) generally assume that landslide
debris is ultimately transportable by rivers. This is an assumption,
however, that our results do not necessarily support. One obvious question
raised by our observations therefore is how channels that are blocked with
earthflow debris nevertheless manage to incise bedrock canyons over geologic
timescales. While this question is not answerable with the results of this
study, our observations point to two field-testable hypotheses. Central to
both hypotheses is the fact that earthflows are fundamentally transient in
nature and their activity must punctuate long periods of dormancy (Mackey
and Roering, 2011).
We have observed epigenetic gorges (Ouimet
et al., 2008) at several locations (AL3, unnamed slide between AL3 and AL2)
where active earthflows impinge on Alameda Creek. This observation suggests
that narrow channels in the Franciscan mélange that are buried in debris
may eventually incise around the margins of boulder jams, perhaps during
periods of earthflow dormancy when boulders are no longer being input into
channels. Epigenetic gorge formation might be a particularly important
process in the Franciscan mélange, where the clasts in the mélange,
which accumulate in channels, are typically much harder than the matrix.
Hence, bedrock river incision in the mélange may occur far more
efficiently than boulder transport.
Alternatively, it is possible that large boulders are mobilized during
extremely large but very infrequent flood events (e.g., Cook et al., 2018). In this case,
during periods of earthflow dormancy, infrequent boulder transport events
could result in the slow erosional removal of the valley blockage. In
support of this view, we note that the knickpoint located at Oak Ridge
earthflow, which is currently active, is much steeper than knickpoints
located at dormant earthflows in the region (Figs. 5 and 6). This
suggests that over time the valley blockages may diffuse due to sediment
transport. The process of boulder transport in this scenario would likely be
aided by abrasion (and hence size reduction) of boulders in place from
suspended sediment (Schumm and Stevens, 1973).
Data availability
River elevation long profile and valley width data used in this study can be downloaded from 10.5281/zenodo.3403345 (Finnegan, 2019a).
Measured boulder size distribution data used in this study can be downloaded from 10.5281/zenodo.3403338 (Finnegan, 2019b).
Earthflow displacement data used in this study can be downloaded from 10.5281/zenodo.3403350 (Finnegan, 2019c).
USGS data for Arroyo Hondo used in this study can be downloaded from https://waterdata.usgs.gov/nwis/inventory/?site_no=11173200&agency_cd=USGS (U.S. Department of the Interior/U.S. Geological Survey, 2019a).
USGS data for the Eel River used in this study can be downloaded from https://waterdata.usgs.gov/nwis/inventory/?site_no=11475000&agency_cd=USGS (U.S. Department of the Interior/U.S. Geological Survey, 2019b).
NASA/JPL UAVSAR data used in this study may be downloaded through their website (https://uavsar.jpl.nasa.gov/, NASA, 2019).
NF designed the study with input from JR and GB, JR supervised the mapping of landslides along the Eel River in a previous study, NF and AN performed the mapping of landslides along Arroyo Hondo and Alameda Creek, KB collected boulder size distribution data, AH processed the InSAR data, NF processed the topographic and hydrological data, and AN performed the mapping and deformation analysis of Oakridge earthflow in a previous study. All authors contributed to the writing of the paper.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
This work was supported by a National Science Foundation (NSF) Graduate
Research Fellowship awarded to Alexander L. Nereson and the Geomorphology and Land Use
Dynamics Program of NSF (EAR-1658800 and EAR-1613122 to Noah J. Finnegan). We thank Ben Brooks, Zhen Liu, and the UAVSAR flight and data processing teams for their help with
acquiring and processing the data. Part of this research was sponsored by
the NASA Earth Surface and Interior focus area and performed at the Jet
Propulsion Laboratory, California Institute of Technology. Alexander L. Handwerger's research
was supported by an appointment to the NASA Postdoctoral Program at the Jet
Propulsion Laboratory, administered by Universities Space Research
Association under contract with NASA. We thank the San Francisco Public
Utilities Commission and Russ Fields for site access. In addition, we thank
Roman DiBiase, Jeff Prancevic and Isaac Larsen for comments that improved
and, in some cases, fundamentally reshaped aspects of this paper.
Financial support
This research has been supported by the National Science Foundation, Directorate for Geosciences, Division of Earth Sciences (grant nos. EAR-1658800 and EAR-1613122).
Review statement
This paper was edited by Paola Passalacqua and reviewed by Isaac Larsen and Jeffrey P. Prancevic.
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