Geology Reference
In-Depth Information
Rates of erosion
are commonly defined in
terms of lowering of the
bedrock
surface, and
have previously been portrayed as a function of
the
local topographic relief
(altitudinal differ-
ence within an area that is typically smaller
than the entire catchment). Owing to the
greater potential energy of elevated areas with
respect to the local base levels (as represented
by adjacent valley bottoms) and owing to the
generally steeper slopes that prevail in regions
of high relief, the traditional view is that high
relief promotes high erosion rates. Measured
and estimated sediment fluxes out of drainage
basins with slow to moderate erosion rates
(
<
0.5 mm/yr), but lying within a similar temper-
ate climatic regime, display a linear relation-
ship to relief (Ahnert, 1970) (Fig. 7.3A). A
similar near-linear relationship of hillslope
angle to erosion rate is apparent at slow to
moderate rates and lower hillslope angles (Fig.
7.3B). As erosion rates increase, however, they
become increasingly independent of slopes or
relief (Montgomery and Brandon, 2002). In
particular, nonlinear erosion-rate increases can
be related to the presence of hillslopes hover-
ing near the threshold angle for landsliding,
such that small changes in slope angle are pre-
dicted to produce large changes in erosion
rates (Fig. 7.3B).
When considering the basin- to range-scale
evolution of the land surface, long-term, aver-
age rates of erosion, deposition, and uplift are
more important than are short-term rates.
Nonetheless, given the techniques available,
most measured sediment fluxes out of basins
sample only short intervals of time. In the
context of the well-known glacial-interglacial
climatic variations of Quaternary times
0.5
Relief & Erosion
in Mid-Latitude
Catchments
0.4
0.3
0.2
0.1
0.0
0
1
2
3
A
Mean relief (km)
1.0
Erosion rate ~
S
[1-(S/S
c
)
2
]
S: slope
0.8
0.6
0.4
Non-Linear
Slope
Dependence
0.2
0.0
0
5
10
15
20
25
30
35
B
Mean slope (°)
10
Soil or
Rock
S
trength
Rapid,
Threshold-
Dependent
Erosion
8
6
4
2
Tectonics
& Climate
angle. At rates
<
0.5 mm/yr, rates are approximately linearly
related to slopes, but, as slopes steepen, rates become
increasingly independent of slope. This response is
consistent with a nonlinear slope dependence in which
rates trend toward infinity as a threshold slope is
approached. C. In rapidly eroding ranges, relief is a poor
predictor of erosion rate. This decoupling is attributed to
the role of threshold slopes in accelerating erosion rates.
Modified after Ahnert (1970) and Montgomery and
Brandon (2002).
Ahnert trend
0
0
1
2
C
Mean local relief (km)
Fig. 7.3
Erosion rates as a function of relief
and hillslope angles.
A. Topographic relief versus erosion rate, showing a linear
correlation for mid-latitude, temperate catchments with
rates
<
0.5 mm/yr. B. Erosion rate as a function of hillslope
