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increasing resisting power
increasing stream power
sediment size
roughness
stream slope
m/km
0.01 Fine
500 Coarse
Steep 100
0.08 Rough
Flat 0.1
0.02 Smooth
steepening
increasing
degradation
aggradation
Balance of
Stream Power
Bedload
Stream Discharge
Fig. 2.10 Schematic representation of the threshold of critical power as a balance between eroding and
resisting forces.
An increase in stream slope or discharge, or a decrease in sediment load, sediment caliber, or bed roughness, will
move the system toward erosion of its bed. Modified after Bull (1991).
aggrades a bit, paired terraces (correlative terraces
preserved on both sides of the river) with consid-
erable downstream continuity can be formed
(Fig.  2.11). If a river is degrading through allu-
vium and is also switching its course back and
forth within a valley during degradation, it can
create terraces that are unpaired (no  correlative
terrace on the opposite side of the river). The
downstream geometry of such terraces may be
hard to reconstruct (Merritts et al. , 1994), because
unpredictable successions of terraces are pre-
served at any location and an age equivalency
would need to be demonstrated prior to confident
correlation among terrace remnants. Such terraces
can provide useful markers for deformation that is
contained within an individual terrace remnant,
but they are much less practical for examining
broader patterns of tectonic deformation.
A river incising into bedrock can create a
bedrock terrace or strath terrace (Plate 1C). Such
bedrock incision typically occurs within or imme-
diately adjacent to mountains, where variations
in bedrock resistance to erosion are common
along the river's course. Across less resistant
bedrock, stream power might tend to be lower
due to some combination of river widening and
gradient decrease. Even in an equilibrium condi-
tion without tectonism, the river gradient in a
bedrock river will be more variable than in an
alluvial river. Thus, as with unpaired degrada-
tional terraces, strath terraces provide useful
local geomorphic markers, but are typically not
as useful in the documentation of regional
deformation patterns. This restriction is all the
more true due to the limited downstream extent
of such strath surfaces, many of which are only
100 m or so in extent when cut into resistant
bedrock. Clear exceptions occur both when the
underlying bedrock is weak, so that rivers can
more readily bevel regionally extensive terraces,
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