Geology Reference
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that are being systematically lowered during
large discharges. Similarly, river profiles are
assumed to display sustained, predictable
decreases in channel slope in a downstream
direction (Fig. 8.5) in the face of steady rock
uplift and invariant lithology. Although the
presence of alluvial fills (commonly resulting
from climate change) is recognized as impeding
vertical bedrock incision along a river channel,
such fills can be remobilized and rapidly incised
by the same rivers that deposited them. As a
consequence, the downstream, bedrock profile
of such rivers is expected to be reoccupied
following incision of any alluvial fill.
In contrast to fluvially aggraded fills that
incrementally accumulate grain by grain,
landslides can deliver large volumes of
disaggregated bedrock to the river channel in a
geological instant. The resultant mass commonly
dams the river and a lake develops as the river
ponds upstream of the dam. Whereas, for a few
major rivers, landslide dams are removed rapidly
by powerful outburst floods (Burbank, 1983),
for most rivers, some blocks within any large
landslide will exceed the river's transport
capacity. As a consequence, the landslide dam
cannot be readily eroded and instead remains as
an obstruction for centuries or millennia (Korup,
2006). Upstream of the dam, fluvial and
lacustrine sediments commonly aggrade to the
height of the outlet across the dam. Within the
dam itself, a narrow, steep rapid that is lined
with remnant, untransportable boulders is
developed (Fig. 8.6A). As long as the landslide
dam persists, some of the former bedrock
channel will be protected from erosion. Along
some rivers in eastern Tibet, for example, over
80% of the bedrock profile is shielded from
erosion by a succession of landslide dams and
associated upstream aggradation (Fig. 8.6B
and C). Thus, the persistence of landslide dams,
their recurrence intervals, and their spacing
along a river network determine key aspects
of the long profile of the river, the amount of
bedrock that is exposed to erosion, and the
long-term rate of bedrock erosion (Ouimet
et al
.,
2007). The potential imprint of landslides instills
a cautionary note on the unfettered interpretation
of river profiles derived from DEMs: high
Id
ealized Slope-Area Data
A
c
Hillslope/
Colluvial
Channel
Alluvial
Channel
Bedrock Fluvial
Channel
A
log drainage area
10
0
Digital Elevation Model Data
θ
= 0.54
k
s
= 29
A
c
10
-1
Hillslope/
Colluvial
Channel
Bedrock Fluvial
Channel
B
10
5
10
6
10
7
log drainage area (m
2
)
Fig. 8.5
Channel characteristics derived from DEMs.
A. In idealized log slope-log area space, channels with
constant concavity define a straight line with slope
q
,
equal to −
m
/
n
. The fluvial channel begins at a critical
catchment area,
A
c
. B. Example of slope-area data
extracted from a 25-m DEM. This catchment is
experiencing rock uplift of ~4 mm/yr, but shows average
concavity. Despite rapid uplift, steepness is modest, at
least in part due to very weak local rocks, which also
promote a smaller-than-average critical area,
A
c
. Modified
after Duvall
et al.
(2004).
±
For many catchments, concavities of 0.5
0.2 are
typical. Interestingly, except for rivers in a tran-
sient state, this concavity is largely independent
of uplift rate, rock type, or precipitation regime,
unless these factors exhibit strong variability
within a catchment. Hence, departures from nor-
mal concavity, spatial variations in steepness, and
perturbations to the linear data array in log area-
log slope space can steer us to parts of a catch-
ment where tectonic processes of interest may be
disrupting normal channel profiles.
Caveats on river profiles
In rapidly eroding ranges such as the Southern
Alps or the Himalaya, river channels are typically
conceptualized as primarily bedrock channels
