Geology Reference
In-Depth Information
erosion or  aggradation. As stream power
increases, the energy supply at the channel bed
is also interpreted to increase, such that more
energy becomes available to overcome friction,
erode the bed, or transport sediments. The
term unit stream power or specific stream
power represents the stream power per unit
area of the bed and is  numerically related to
bed shear stress and mean velocity. It can be
easily imagined that changes in stream power
will affect the behavior of a river, such as
whether it is aggrading or degrading (Whipple
and Tucker, 1999). However, other variables,
such as the sediment load, the caliber of the
sediment, and the roughness of the bed, also
influence the aggradational or degradational
state of the river (Fig. 2.10). Increases in load,
caliber, or roughness are commonly interpreted
to increase the resistance of the river bed to
erosion, because larger loads require a higher
expenditure to transport and greater roughness
dissipates more energy through turbulence.
A river that is neither aggrading nor degrading
can be considered to be in equilibrium (Bull,
1991) and to be at the threshold of critical
power (Fig. 2.10). At this threshold, the stream
power is just sufficient to transport the sedi-
ment load that is being supplied from upstream,
and the height of the bed remains constant. In
general, if other factors are held steady,
increases in river  slope or in discharge, or
decreases in bed roughness, sediment load, or
sediment caliber, will cause the river to cross
the threshold of critical power and begin to
erode its bed. In contrast, changes in the oppo-
site sense will push the river into an aggrada-
tional mode. The concept of a threshold of
critical power has been usefully applied to the
interpretation of the genesis of river terraces,
because it indicates the potential linkages
among different variables  and suggests how
changes in climate or tectonics could cause the
river to switch from aggradation to degradation,
or vice versa.
River terraces are common examples of
preserved, sloping geomorphic features. Two
classes of river terraces are typically defined:
aggradational (or constructional or fill), and
degradational (or erosional or cut or strath ).
proxy for water level:
topset-foreset contact
Gilbert
Delta
topset
foreset
bottomset
coarse
medium
fine
Fig. 2.9 Internal bedding geometries in a simple delta.
The contact between the foreset and topset beds closely
approximates the lake level or sea level at the time of
delta growth.
permit reliable correlation of deltas formed in
the same interval.
River terraces
All of the previously discussed markers provide
a horizontal reference frame for assessing defor-
mation. Even if the geomorphic evidence for a
displaced shoreline feature is discontinuously
preserved across the area of interest, the former
geometry of the pristine feature is known to be
horizontal, such that vertical displacements can
be confidently calculated. If the timing of the
surface cannot be determined, one can still
have considerable confidence in the relative
displacements of points, and hence determine
vertical displacement field (but not the dis-
placement rate ). When geomorphic features
that were not originally horizontal are used as
markers, care must be taken to ascertain the
gradient and geometry of the feature prior to
offset. For example, if a change in gradient is
used to define tectonic warping, one needs to
be confident that these gradient changes are
not natural ones resulting from some non-tec-
tonic cause, such as variable resistance of bed-
rock to erosion or the normal downstream
gradient of a river.
The term stream power refers to the rate of
expenditure of potential energy per unit length
of stream (Box 2.2) and is proportional to the
slope of the water surface and to the river
discharge. Analysis of stream power, including
spatial and temporal changes in power, pro-
vides one perspective on the causes of river
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