Geology Reference
In-Depth Information
local
erosion
at a point
eroded volume
modern
sediment
discharge
present surface
present volume of sediment fill
Erosion Volumes and Rates
original surface at time 0
Fig. 7.4
Approaches to estimating erosion rates.
If the original surface topography can be reconstructed, then the missing volume of material can be determined by
subtracting the present-day topography from the pre-erosion topography. If the age of that initial surface is known,
then a mean erosion rate can be calculated. Similarly, if the volume of sediment in a basin and the age span of its
filling are known, and if the size of the area contributing sediment to the basin can be estimated, a mean erosion
rate can be approximated. Alternatively, the modern rate of sediment discharge and the contributing area can be
used to compute a mean erosion rate.
(Fig. 2.3), sediment fluxes out of basins
should be expected to occur in irregularly
spaced pulses. Although the timing of major
episodes of erosional or sediment discharge
may vary between nearby basins, regional cli-
mate change more commonly modulates sedi-
ment fluxes and creates a coherence among
nearby catchments. Such changing sediment
pulses could directly relate to precipitation
variations (Goodbred and Kuehl, 1999) or to
the stability of hillsides influenced by climati-
cally modulated vegetation change (Leeder
et al.
, 1998).
Several approaches can be used to estimate
erosion rates (Fig. 7.4). An average denudation
rate can be readily computed with three types
of data: the volume of eroded material, the area
from which that material was derived, and the
duration of erosion. The volume of material
eroded from a catchment can be estimated
using direct measurements of erosion within
the catchment itself, gauging the sediment flux
out of the catchment, or by assessing the vol-
ume of sediment stored in a basin to which
that catchment is tributary (Fig. 7.4). These sed-
iment volumes have to be converted to rock
volumes with appropriate corrections for den-
sity differences. Whereas direct measures of
erosion rates in the catchment are informative,
rates at individual points must be integrated
across the entire catchment. Commonly, basin-
wide estimates with large and unknown uncer-
tainties result from extrapolation of a few point
measurements. If, over the span of several
years, for example, the total, time-averaged
sediment flux, including the dissolved load, out
of a catchment could be measured, this flux
would provide a good estimate of average,
short-term denudation rates in many basins.
Similarly, if the volume and age of sediment
stored in a basin are known, if it can be shown
that significant volumes of sediment did not
bypass the basin
en route
to another basin, and
if the tributary catchments throughout deposi-
tion can be reliably reconstructed, then a long-
term mean erosion rate for the tributary
catchments can be determined (Métivier
et al.
,
1999). Measurement of both modern fluxes and
stored sediment volumes are useful, because
they commonly encompass different temporal
intervals.
Sedimentary fluxes in rivers
The sediment load of a river can be partitioned
into the bedload, suspended load, and dissolved
load. If the average contribution of each compo-
nent over time is known, then mean denudation
rates can be calculated. Whereas the highest bed-
load and suspended-load fluxes typically occur
during high water discharge, the highest solute
concentrations typically occur during low flows
