Quantifying human impacts on rates of erosion and sediment
transport at a landscape scale
Lucas Reusser1, Paul Bierman1*, and Dylan Rood2,3
Rubenstein School of Environment and Natural Resources, and Department of Geology, University of Vermont, Burlington, Vermont
05405, USA
2
Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
3
Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550, USA
1
ABSTRACT
Establishing background (geologic) rates of erosion is prerequisite to quantifying the impact
of human activities on Earth’s surface. Here, we present 10Be estimates of background erosion
rates for ten large (10,000–100,000 km2) river basins in the southeastern United States, an
area that was cleared of native forest and used intensively for agriculture. These 10Be-based
rates are indicative of the pace at which the North American passive-margin landscape eroded
before European settlement (~8 m/m.y.). Comparing these background rates to both rates
of post-settlement hillslope erosion and to river sediment yields for the same basins, we find
that following peak disturbance (late 1800s and early 1900s), rates of hillslope erosion (~950
m/m.y.) exceeded 10Be-determined background rates more than one-hundred fold. Although
large-basin sediment yields during peak disturbance increased 5–10× above pre-settlement
norms, rivers at the time were transporting only ~6% of the eroded material; work by others
suggests that the bulk of historically eroded material remained and still remains as legacy
sediment stored at the base of hillslopes and along valley bottoms. Because background erosion rates, such as we present here, reflect the rate at which soil is generated over millennial
time scales, they can inform and enhance landscape-management strategies.
*E-mail: [email protected]
1- Roanoke
2- Dan
3- Neuse
4- Pee Dee
5- Wateree
6- Saluda
7- Savannah
8- Oconee
9- Ocumulgee
10- Chattahoochee
VA
1
N
38oN
2
3
4
NC
- Blue Ridge
- Mid-basin
- Outlets
5
7
Valley and Ridge
n
lai
lP
Pie
d
100 km
ta
GA
on
t
Blue Ridge
9 8
AL
SC
m
10
6
Co
as
INTRODUCTION
Quantitatively determining background rates
of erosion remains a difficult but critical task.
Knowing such rates places the impact of human
activities on natural process rates in context
(National Research Council, 2010) and is thus
critical for environmental decision making, such
as setting allowable levels of suspended sediment
(Whiting, 2006). Traditional approaches used to
quantify the mass of sediment moving through
fluvial systems, such as contemporary sedimentyield data, do not always reflect long-term, background rates of erosion (Trimble, 1977).
Human activities, including forest clearance
for wood products and agriculture, can dramatically elevate the pace at which sediment moves
down slopes and possibly into river systems
(Hooke, 1994). This can increase sediment
yields well above natural levels (Meade and
Trimble, 1974); yet, dams and the reservoirs
they create trap much of this sediment before
it moves downstream to the oceans (Walter and
Merritts, 2008; Syvitski et al., 2005). If erosion
following human-induced landscape disturbance outpaces the rate at which streams can
access or transport the material fed to them, sediment-yield data represent neither natural erosion nor the maximum rate of upstream erosion.
Instead, sediment-yield data may represent the
connectivity of slopes with channels; in most
settings, much of the material eroded because
of human-induced change remains trapped on
the landscape (Haggett, 1961; Walling, 1983;
Wilkinson and McElroy, 2007; Walter and
Merritts, 2008). Furthermore, sediment-yield
records are commonly short (years to decades)
and thus may miss large volumes of sediment
delivered to rivers during high-magnitude, lowfrequency events (Kirchner et al., 2001).
The broad low-slope southern Piedmont
region of the Appalachian Mountains (eastern
United States) (Fig. 1), with its subdued topography, humid-temperate climate, and clay-rich
soil, was subjected to logging and intensive
European agricultural practices beginning in the
1700s (Trimble, 1974). It is possible that upland
soil erosion began centuries earlier, coincident
with the advent of maize agriculture by Native
Americans (Stinchcomb et al., 2011). The head-
79oW
Figure 1. Map of southern Appalachian Piedmont along southeastern passive margin of
North America. River basins 1–10 are those of Trimble (1977). Blue Ridge (star), mid-basin
(circle), and outlet (triangle) denote locations of in situ 10Be sample sites within each catchment. Insets show location of map and physiographic provinces mentioned in text. Modified
from Trimble (1977, his figure 1). VA—Virginia; NC—North Carolina; SC—South Carolina;
GA—Georgia; AL—Alabama.
GEOLOGY, February 2015; v. 43; no. 2; p. 171–174; Data Repository item 2015064 | doi:10.1130/G36272.1 | Published online 7 January 2015
©
2015 Geological
Society
America.
permission to copy, contact [email protected].
GEOLOGY 43 | ofNumber
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171
waters of the largest catchments (>10,000 km2)
originate in the rugged Blue Ridge province.
Beginning in the 1700s and peaking in the early
1900s, land clearance for timber and for cotton
and tobacco production increased dramatically.
This land use resulted in widespread erosion of
Piedmont hillslopes to an average depth of ~180
mm and triggered aggradation of valley bottoms
and toe slopes (Costa, 1975; Meade and Trimble, 1974; Phillips, 1992, 2006; Trimble, 1974;
Trimble, 1977; Fig. 2).
Sediment dynamics on the passive eastern
margin of North America have been well studied to understand the erosional consequences
of intensive agricultural practices (Costa, 1975;
Trimble, 1977). Contrasting the depth of soil
erosion resulting from logging and peak cotton
and tobacco production during the early 1900s
with the sediment yields recorded on Piedmont–Coastal Plain rivers clearly established
the discordance between hillslope erosion and
sediment yields in these large, low-gradient
river systems (Trimble, 1977). For example,
the degree of soil truncation in the ten basins
examined in this study (Trimble, 1974, 1977)
suggested an area-weighted hillslope erosion
rate of ~950 m/m.y. In contrast, area-weighted
sediment yields expressed as erosion rates at
the river outlets during the same time were ~53
m/m.y. (Trimble, 1977, Table DR1 in the GSA
Data Repository1), so the sediment delivery
ratio (the ratio of suspended sediment flux at
basin outlet to the upstream erosion rate) was
only ~6% (Figs. 2 and 3). Clearly, most of the
material shed from slopes was not transported
by mainstem rivers during the time of intensive
Piedmont land use; rather, much of the eroded
hillslope soil was stored as alluvial and colluvial
deposits along low-gradient Piedmont valley
bottoms (Meade and Trimble, 1974; Trimble,
1977). By analogy with the mid-Atlantic states
(Walter and Merritts, 2008), sediment was likely
retained in lower-order valley bottoms by mill
dams, which were common in the Piedmont
(Wegmann et al., 2012).
These findings challenged the long-held
assumption that the mass of material eroded
from hillslopes was in equilibrium with the
mass of sediment carried by rivers over the short
term, an assumption of steady-state sediment
production and export (Dole and Stabler, 1909;
Judson and Ritter, 1964; Menard, 1961). Prior
to the 1980s, there were no techniques capable
of reliably quantifying background rates of erosion over geologic time scales. In the absence of
this information, there was no way to quantify
the degree to which human land-use practices
increased erosion above background rates.
Here, we use in situ cosmogenic 10Be measured in present-day river sediment samples (n
= 24) to infer background drainage-basin erosion rates from low-gradient southern Appala-
chian Piedmont catchments where the effect of
land use on soil erosion and sediment yield is
well known (Trimble, 1977). Concentrations
of in situ–produced 10Be measured in the sand
fraction of fluvial sediments are used to estimate spatially averaged, millennial-scale rates
of sediment production and landscape erosion
(Portenga and Bierman, 2011). The concentration of 10Be is homogenized in the upper ~1 m of
Earth’s surface as hillslope materials are stirred
by bioturbation (Jungers et al., 2009) making
erosion rate estimates insensitive to all but the
most deeply penetrating forms of mass wasting
(Niemi et al., 2005). Thus, in most instances,
erosion rates modeled from 10Be measurements
in modern river sediments still record the isotopic signature of longer-term hillslope erosion
(103–104 yr) and constitute a useful metric for
comparison to human-induced rates of erosion
(von Blanckenburg et al., 2004). Cosmogenic
10
Be data, in concert with other data, allow us to
quantify the erosive effects of human land-use
practices in a region with a profound and welldocumented history of human disturbance. Specifically, we compare erosion rates calculated
from sediment yields and truncated soil profiles to long-term, background rates of erosion
determined using 10Be. Erosion rates calculated
from 10Be and soil truncation are independent of
grain size; however, sediment yields are based
on fine-grained suspended sediment load, while
10Be background erosion rate integration time (103 to 105 years)
A: Measurement
integration times
Hillslope erosion rate integration time presented in this
study; data sourced from Trimble (1977)
Land clearance
Long-term background conditions
~10 ka
Time Line
Break in time-scales
Sediment yield-derived erosion rate integration time
presented in this study; data sourced from Trimble (1977)
Assumed limited storage of sediment across
landscape prior to agricultural disturbance.
Peak
agricultural
use
B: Land clearance
through time
(Zoomed inset)
1800
1900
Today
Pervasive
Increased
agricultural
soil conservaland clearance.
tion.
Break in time-scales
Assumed variability in natural long-term
landcover conditions.
Assumed variability in hillslope erosion rates. Over
long-periods, mass flux off hillslopes in approximate
equilibrium with mass of sediment carried by streams.
short-term historic conditions
Maximum
hillslope erosion.
Sediment loads
limited by slope
connectivity to streams.
C: Landcover
conditions
Decreased
hillslope
erosion.
D: Hillslope
conditions
Reduced
sediment
loads.
E: Sediment loads
carried by streams
Vast quantities
Some legacy
of sediment stored
sediment now
in valley bottoms. stored in reservoirs.
Figure 2. Change over
time in conditions of the
study area. A: Integration
times of data. B: Land
clearance. C: Land cover.
D: Hillslopes. E: Stream
sediment loads. F: Storage of legacy sediment.
Horizontal break is longversus short-term conditions in panels B to F
represented by separate
boxes.
F: Storage of legacy
sediment on
landscape
1
GSA Data Repository item 2015064, Table DR1 (sample locations, basin characteristics, and isotopic data), is available online at www.geosociety​.org​/pubs/ft2015.htm,
or on request from editing@geosociety​.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
172www.gsapubs.org | Volume 43 | Number 2 | GEOLOGY
104
NE
SW
All Basin
Averages
Rate (m/m.y.)
103
Post-colonial
upland erosion
(Trimble,1977)
Post-colonial
sediment yield
(Trimble, 1977)
102
Background
erosion, 10Be
outlet samples
(this study).
101
1
Drainage basins
10. Chattahoochee
9. Ocumulgee
8. Oconee
RESULTS AND DISCUSSION
Background, landscape-scale erosion rates,
calculated from the concentration of 10Be in
river sediment, are much lower than rates of both
soil erosion and sediment transport during peak
land use. During the early 1900s, aerially averaged rates of hillslope erosion (~950 m/m.y.)
exceeded 10Be-derived background erosion
7. Savannah
METHODS
Samples of active-channel sediment or recent
overbank deposits were collected and field
sieved to a grain-size fraction of 1000–250 µm
for in situ 10Be analysis. Samples from the outlets of the ten large Piedmont drainages correspond directly to those presented in Trimble
(1977). Blue Ridge and mid-basin samples
were collected from within a subset of the ten
large basins (Table DR1). Geographic statistics
were calculated in ArcGIS software (www.esri.
com) using 1/3 arcsecond (~28 m) digital elevation models (downloaded from www.nationalmap.gov). Quartz was purified using selective
acid etching (Kohl and Nishiizumi, 1992), and
Be was extracted using HF dissolution and ion
exchange chromatography (Corbett et al., 2011).
Isotopic measurements were made on the Lawrence Livermore National Laboratory accelerator mass spectrometer (Rood et al., 2013). Errors
in nuclide concentrations include only 1s ratio
measurement uncertainties. Measured ratios of
10
Be/9Be were normalized to the 07KNSTD3110
standard (Nishiizumi et al., 2007) with an
assumed ratio of 2850 × 10-15 and corrected
using process blanks run with each batch of 11
samples. 10Be erosion rates were modeled with
the CRONUS online calculator (http://hess.ess.
washington.edu, version 2.2) and Lal/Stone scaling factors (Portenga and Bierman, 2011).
6. Saluda
5. Wateree
4. Pee Dee
3. Neuse
2. Dan
1. Roanoke
reservoir sedimentation rates reflect primarily
coarser suspended load and bed load.
rates (~8 m/m.y.) by more than one-hundred
fold (Fig. 3; Table DR1). Even with an aerially
averaged sediment delivery ratio of only ~6%,
streams during the early 1900s were still carrying ~7× their long-term average or “equilibrium” sediment mass as determined from 10Be
measurements (Figs. 2 and 3).
The bulk of the eroded soil (an estimated 25
km3; Trimble, 1974) went into storage in valley
bottoms and on toe slopes (Meade, 1982; Trimble, 1977). Because today much of the material
in these deposits (legacy sediment) is not directly
adjacent to river channels, it is difficult to estimate
how much time will be required to remove the
displaced hillslope material from the Piedmont
and eventually transport it offshore, although it is
likely to take millennia (Jackson et al., 2005). As
a consequence, the load of sediment carried by
these large rivers and their tributaries will remain
elevated (Meade, 1982; Phillips, 2006) (Fig. 2).
Furthermore, nearly all Piedmont drainages contain dams with sediment trap efficiencies of up
to 95% (Brune, 1953). Thick sediment deposits
within the dam reservoirs will remain trapped
during all but the largest floods.
In contrast to the Piedmont-dominated lowerslope outlet samples, 10Be results indicate that
the more rugged Blue Ridge province, included
in eight of the ten large basins, naturally erodes
faster (8.2 ± 3.2 versus 14.6 ± 5.4 m/m.y.,
respectively; Table DR1). The difference is
significant at a = 0.05. Samples collected from
midway down six of the ten streams indicate an
average erosion rate of 9.3 ± 4.6 m/m.y., suggesting that sediments mix predictably as they
travel from Blue Ridge headwater regions to
their Piedmont outlets.
Due to effective soil conservation measures
across the southern Appalachian Piedmont over
the last century, sediment yields in many tributary streams have been substantially reduced.
For example, deposition of sediment between
GEOLOGY | Volume 43 | Number 2 | www.gsapubs.org
Figure 3. Summary of erosion rates for large-scale
catchments presented
and discussed in this
paper. Refer to Figure 1
for locations. Dark gray
(rates of hillslope erosion) and light gray (sediment yield expressed
as rates of erosion) bars
from data presented by
Trimble (1977). Black bars
represent in situ 10Be back­
ground erosion rate estimates. Note log scale.
A.D. 1910 and 1934 in the Lloyd Shoals Reservoir, completed in 1910 along a tributary of
Georgia’s Ocmulgee River, suggests a sediment
yield equivalent to ~73 m/m.y. of basin-wide
erosion (Meade and Trimble, 1974). Between
1967 and 1972, following extensive soil conservation improvements within the watershed,
sediment deposition in the reservoir dropped to
a basin-wide erosion equivalent of ~10 m/m.y.,
a value which matches well the 10Be results
from this area (11 m/m.y., sample SAP50; Table
DR1). These data suggest that soil conservation
practices can reduce sediment yields so that
they are closer to the rate at which the landscape
erodes naturally; however, some fine-grained,
suspended sediment likely moved through the
reservoir and downstream. If so, sediment yields
remain elevated over background rates.
These findings unequivocally demonstrate the
effects that human land-use practices have had on
natural systems of sediment generation and erosion along the southern Appalachian Piedmont, a
condition that has been studied extensively worldwide, yet rarely quantified (Hicks et al., 2000;
Marsh, 1882; Syvitski et al., 2005). Background
erosion rates, determined through the measurement of in situ–produced 10Be, provide the context
from which to assess nearly all other measures of
erosion germane to a human time scale, and thus
hold the potential to inform a variety of landscape
management strategies. Such isotopic estimates
could serve as benchmarks for establishing sediment Total Maximum Daily Load (TMDL) that
are consistent with natural rates of sediment supply to rivers, and they can be used to determine
whether rates of soil loss are sustainable (Montgomery, 2007). Because 10Be integrates over
time and space scales appropriate for understanding pre-settlement background rates of erosion
and sediment supply, such measurements place
human impacts in context, making 10Be an important tool for informing landscape management.
173
ACKNOWLEDGMENTS
We thank M. Jungers, W. Hackett, and J. Duxbury
for field assistance. Support was provided by National
Science Foundation grant EAR-310208 and Department of Defense grant DAAD19-03-1-0205. We
thank R. Hooke, T. Dunne, K. Wegmann, and D. Merritts for reviews that improved this paper.
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Printed in USA
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