Geology Reference
In-Depth Information
River Response to Flexure
60
high valley slope and sinuosity
40
P = 1.74
S
p
= 5.56
S
c
= 3.20
20
P = 1.52
S
p
= 3.67
S
c
= 2.41
P = 3.08
S
p
= 8.41
S
c
= 2.71
P =1.23
S
p
= 1.85
S
c
= 1.50
P = sinuosity
S
p
= projected channel slope (x10
-4
)
S
c
= channel (thalweg) slope (x10
-4
)
0 0 0 0
km
A
10
zone of steepened
valley slope
sinuosity (P)
channel slope
valley slope
8
6
Fig. 8.20
Fluvial response to
localized uplift.
A. Longitudinal profile of the San
Antonio River and associated changes
in valley slope (
S
p
), river slope (
S
c
),
and sinuosity (
P
). Modified after
Ouchi (1985). B. Plot of valley
gradient, channel slope, and sinuosity
along the San Antonio River. Through
the reach where the valley slope more
than doubles, the sinuosity also
doubles, whereas the channel slope is
almost unchanged.
increased
sinuosity
4
2
neglibible change
in channel slope
0
10
20
30
40
50
60
70
B
Distance (km)
in each river (Fig. 8.21). Both rivers narrow
significantly across the zone of rock uplift.
In fact, a tight spatial correspondence exists
between the rate of rock uplift (as inferred
from deformed terraces; Fig. 7.25) and the
magnitude of channel narrowing. Incision
rates as high as 12 mm/yr broadly mimic the
magnitude of narrowing. In contrast to their
similar response of narrowing, the Bakeya
River becomes steeper across the zone of
most rapid uplift, whereas the Bagmati River
shows no significant steepening across this
same zone. Although the cause of this contrast
is unknown, the Bagmati has a far larger
upstream source area and carries a con-
siderably greater sediment load. Possibly, the
channel slope is adjusted primarily to carry
the sediment load. Alternatively, because the
maximum incision rate along the Bagmati is
30% lower than that along the Bakeya, perhaps
the Bagmati's erosive capability is increased
sufficiently via channel narrowing alone. This
interpretation would be consistent with the
observation that the Bakeya channel steepens
only where its incision rates exceed about
10 mm/yr (Fig. 8.21).
Given that both narrowing and steepening
of the channel can enhance incision rates, does
one take precedence over the other, or do both
typically occur synchronously? The data from
the Nepalese foreland (Fig. 8.21) suggest that
narrowing can occur without steepening. Such
