Geology Reference
In-Depth Information
A
transi-
tional
meandering
braided
1.3
1.2
1.1
Slope-Dependent
Sinuosity
1.0
0
0
.005
.01
.015
.02
Slope
B
slope ~ 0.012
1.3
Fig. 8.17
Slope-driven changes in river
sinuosity: stream-table results.
A. Channel sinuosity on a stream table as
a function of slope. Sinuosity increases
until a threshold is crossed at a slope of
~0.013. Modified after Schumm and Khan
(1972). B. Sinuosity as a function of
stream power for different discharges.
Note that the transition from higher to
lower sinuosity occurs at the same
average slope for all three discharges.
Modified after Edgar (1973).
0.003
m
3
/s
1.2
1.1
m
3
/s
0.009
1.0
Stream Power v. Sinuosity
m
3
/s
0.006
0.10
1.0
Stream Power (watts/m)
lessened in comparison to a shorter, steeper,
and straighter channel.
In another stream-table experiment, a kink
fold was created across an established chan-
nel midway along its length (Ouchi, 1985).
The fold directly deformed only the middle
third of the stream table, and fold growth was
mimicked by incrementally increasing the
height of the fold crest. Such growth caused
an increase in the valley slope on the down-
stream side of the fold axis, whereas it dimin-
ished the slope on the upstream flank of the
fold. When folding was superimposed on a
pre-existing moderately sinuous river pattern,
the primary response to the uplift was an
increase in the thalweg sinuosity in the down-
stream part of the uplift, where the slope
was steepened (Fig. 8.18A). The elevation of
the thalweg itself increased in this uplifted
area, but the increased sinuosity compensated
for the steeper slope of the “valley” bottom.
Thus, a convexity developed in the valley
slope profile, but little significant aggradation
or degradation occurred at the fold crest or
on its downstream limb. Upstream of the
uplift, deposition occurred along the reduced
upstream channel gradient as multiple stable
channels were established (an anastomosing
pattern). With continued uplift, a cutoff formed
downstream of the axis of uplift, and the
channel steepened, straightened, and assumed
an “island-braided” pattern. Folding experi-
ments with a multi-threaded river caused
erosion across the fold crest, where a deep-
ened, single-thread channel bounded by river
“terraces” was created (Fig. 8.18B). Downstream
of the fold axis, the steepened slope and
increased sediment flux (due to erosion of
the fold crest) augmented the pre-existing
braiding. Upstream, however, the decrease in
slope and sediment flux generated alternate
bars with a tendency toward a more sinuous,
single thalweg.
Given the simple observation that sinuosity
tends to increase with increased slope (up to a
threshold), it might be expected that active
deformation could be revealed by map patterns
of meandering rivers showing localized changes
