Geology Reference
In-Depth Information
model is consistent with observations in modern
bedrock channels where bedload covers the thal-
weg during large floods while active lateral ero-
sion occurs on the channel margins several meters
above the thalweg (Turowski
et al.
, 2008). Thus,
when an aggrading fill achieves a new equilibrium
profile, new straths would form on the edges of
the fill. In such a scenario, the height of a strath
above the modern river channel reveals little about
rates of vertical bedrock incision by the channel.
Box 7.2
What are river incision rates measuring?
Changes due to incision in the elevation of a
given reach of a river can modify channel gra-
dients and the stability of adjacent hillslopes.
For landscapes in quasi-steady state, the rate
of river incision is commonly argued to regu-
late the overall rate of erosion. Hence, many
studies focus on defining river incision rates as
interpreted from the age and height of recon-
structed river channels in the past. Although
obtaining reliable ages is commonly a formi-
dable obstacle, a reliable interpretation also
depends on understanding the context of the
incision history. Several key questions must be
answered. Was the incision into bedrock or
into alluvial fill? Was the river profile locally
perturbed at the study site or do the rates
typify adjacent reaches? Is the interval over
which incision has been measured long enough
to yield a representative long-term rate?
We know, for example, that climate changes
commonly drive intervals of aggradation and
degradation. Incision of alluvial fill is far easier
than incision into bedrock. In most circum-
stances, therefore, alluvial incision rates are
unlikely to place useful limits on bedrock inci-
sion or landscape erosion rates. As long as the
bedrock is covered with sediment, no bedrock
incision can occur (Lavé and Avouac, 2001).
Large landslides can dam big river gorges
and cause bedrock incision to cease until
the landslide dam is removed. Whereas
catastrophic dam breaks can occur shortly
after water overtops the dam, more commonly
these dams are slowly eroded. Although such
dams are sedimentary deposits, they typically
comprise large bedrock blocks that erode
much more slowly than does river alluvium.
As a consequence, such landslides retard the
rate of long-term incision even more than do
alluvial fills (Ouimet
et al.
, 2007).
Obvious local perturbations of a river's
gradient occur when rivers cross growing
folds or active faults. A less commonly
recognized perturbation can occur when an
epigenetic
gorge has formed. Such gorges
are created following episodes of alluvia-
tion, landsliding, or glacial advance when a
river is subsequently “let down” on to bed-
rock on the margins of its former valley
(see figure A). Deep, narrow, slot canyons
can be incised into the underlying bedrock.
In comparison to a reach flowing wholly
on bedrock, the epigenetic bedrock reach
commonly is steepened because the down-
stream alluvial fill is more readily removed.
As a consequence, such steepened bedrock
reaches may incise considerably more
rapidly. Wherever a bedrock river channel is
seen to narrow abruptly in the absence of
obvious lithologic or structural controls, the
possibility of an epigenetic origin should
be assessed.
Epigenetic Gorge
A
alluvial
fill
Cross
Profile
bedrock
Long
Profile
steepened reach
cross profile location
A. Epigenetic gorge in cross-section and profile.
