Hydrobiologia 410: 223–240, 1999.
J. Garnier & J.-M. Mouchel (eds), Man and River Systems.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
223
Linking land use, erosion and sediment yields in river basins
D. E. Walling
Department of Geography, University of Exeter, Exeter, EX4 4RJ, U.K.
Key words: erosion, sediment yield, sediment storage, sediment budget, floodplain sedimentation.
Abstract
Results obtained from erosion plots and catchment experiments provide clear evidence of the sensitivity of erosion
rates to land use change and related human activity. Evidence for the impact of land use on the sediment yields
of world rivers is less clear, although examples of rivers where sediment yields have both increased and decreased
in recent decades can be identified. The apparent lack of sensitivity of river sediment loads to land use change
reflects, at least in part, the buffering capacity associated with many river basins. This buffering capacity is closely
related to the sediment delivery ratio of a river basin, in that basins with high sediment delivery ratios are likely
to exhibit a reduced buffering capacity. Investigations of the impact of land use and related human activity on
sediment yields should consider the overall sediment budget of a catchment rather than simply the sediment output.
Information on the sediment budget of a drainage basin is difficult to assemble using traditional techniques, but
recent developments in the application of fingerprinting techniques to establish sediment sources and in the use of
environmental radionuclides, such as caesium-137 and lead-210, to document sediment storage offer considerable
potential for providing such information. Sediment storage within a river basin can give rise to environmental
problems where sediment-associated pollutants accumulate in sediment sinks. The accumulation of phosphorus on
river floodplains as a result of overbank sedimentation can, for example, represent an important phosphorus sink.
The context
The results obtained from erosion plot experiments
and experimental catchment studies in many different
areas of the world provide clear evidence of the sensitivity of erosion rates to land use and related human
activity. Table 1, for example, shows that the change
from natural vegetation to cultivation can increase soil
erosion rates by an order of magnitude or more. Increased rates of soil erosion can be expected to give
rise to increased sediment transport by local streams
and the literature contains results from many experimental catchment studies which show that vegetation
clearance or land use change can again cause increases
in sediment yield of an order of magnitude or more
(cf. Table 2). Abernethy (1990) has similarly reported results from longer-term investigations of several
small reservoir catchments in Southeast Asia which
showed that during the present century annual sediment yields have progressively increased by between
2.5 and 6% per year as a result of land clearance
and subsequent intensification of land use (Figure 1).
Abernethy linked these increases to the rate of population growth in the areas concerned and found that
the ratio of the increase in sediment yield to that
for population was greater than unity and averaged
ca. 1.6:1. Based on these findings, he suggested that
annual suspended sediment yields from river basins
in many developing countries could be expected to
double in about 20 years. If the above generalisations
are coupled with information on the global increase
in population and the global extent of land clearance
and intensification of land use, which indicates, for
example, that the area of the earth’s surface given over
to crop production and livestock grazing has increased
by more than five-fold over the past 200 years (cf.
Buringh & Dudal, 1987), major increases in the sediment loads of the world’s rivers could be inferred (cf.
Douglas, 1967).
The available evidence regarding the impact of human activity and land use change on the sediment
loads of the world’s rivers, is however, less clearcut.
Existing estimates of the global transfer of sediment
from the land surface of the globe to the oceans sug-
224
Table 1. A comparison of soil erosion rates under natural
undisturbed conditions and under cultivation in selected
areas of the world
Country
Natural
(kg m−2 year−1 )
China
<0.20
U.S.A.
0.003–0.30
Ivory Coast
0.003–0.02
Nigeria
0.05–0.10
India
0.05–0.10
Belgium
0.01–0.05
U.K.
0.01–0.05
Cultivated
(kg m−2 year−1 )
15.00–20.00
0.50–17.00
0.01–9.00
0.01–3.50
0.03–2.00
0.30–3.00
0.01–0.30
Based on Morgan (1986).
Figure 1. Trends of increasing sediment yield in several reservoir
catchments in Southeast Asia (based on Abernethy, 1990).
gest that this flux is currently about 20×109 tonnes
per year (cf. Milliman & Syvitski, 1992; Walling &
Webb, 1996), but the extent to which this value has
increased over and above the ‘natural value’ remains
uncertain. In the absence of long-term records of sediment transport for rivers in most areas of the world,
it is necessary to look for other sources of evidence of
the magnitude of this increase. In their classic study
Erosion and Sediment Yield on the Earth, which was
based on sediment yield data assembled from more
than 3600 measuring stations on the world’s rivers,
Dedkov & Mozzherin (1984) used space–time substitution to estimate the extent to which the sediment
yields of the world’s rivers had increased due to catchment disturbance by land use activity. They grouped
the available data from both the mountainous and nonmountainous (plains) regions of the world into large
(>5000 km2) and small (<5000 km2 ) basins and further classified these according to the degree of disturbance by agricultural activity. Category I basins were
largely undisturbed and characterized by either a forest
cover >70% or a cultivated area <30%. Category
II basins exhibited an average degree of disturbance
(i.e. area under forest or cultivation of between 30
and 70%), whereas category III basins were characterized by the greatest degree of disturbance, with an
area under forest of <30% or a cultivated area >70%.
By comparing the average sediment yields of category
I basins, with the equivalent values for category III
basins, it was possible to derive an approximate measure of the magnitude of the increase in sediment yield
associated with land disturbance by human activity
(cf. Table 3). Although the results of this analysis
are necessarily limited by the fact that they represent a simple comparison of mean values of sediment
yield for drainage basins in different categories, and
are therefore heavily dependent on the representativeness of the sample of river basins involved, they
suggest that the sediment yields of many of the world’s
rivers may have increased ca. 5-fold as a result of
disturbance by human activity.
Milliman et al. (1987) provide provide a similar,
although somewhat greater, estimate for the magnitude of the change in the sediment load of the Yellow
River in China since the expansion of agriculture into
the loess plateau, which began about 200 BC. In this
case, an analysis of sediment deposits in the Yellow
Sea was used to estimate that the sediment load of
the Yellow river during the early and middle Holocene
was an order of magnitude less than that associated
with the more recent post-agriculture period.
Other lines of evidence are, however, less conclusive in terms of confirming a substantial increase
in the sediment loads of the world’s rivers as a result
of human activity and land use change. The work of
Tardy et al. (1989), which has attempted to reconstruct global sediment yields over the geological past
(cf. Walling, 1995), indicates that present-day sediment loads are ca. 1.6 times greater than those in the
recent geological past (i.e. the Cenozoic, ca. 33 million years ago) (Figure 2A). However, although the
impact of human activity is invoked as one explanation of this increase, these authors indicate that it may
225
Table 2. Some results from experimental basin studies of the impact of land use change on sediment yield
Region
Land use change
Increase in sediment yield
Reference
Westland, New Zealand
Oregon, U.S.A.
Northern England
Texas, U.S.A.
Maryland, U.S.A.
Clearfelling
Clearfelling
Afforestation (ditching and ploughing)
Forest clearance and cultivation
Building construction
×8
×39
×100
×310
×126–375
O’Loughlin et al. (1980)
Fredriksen (1970)
Painter et al. (1974)
Chang et al. (1982)
Wolman & Schick (1967)
Figure 2. Variation of sediment yields over geological time. Based
(A) on Tardy et al. (1989) and (B) on Degens et al. (1991).
also partly reflect the progressive increase in tectonic
activity since Jurassic times. Studies of long-term sedimentation in water bodies receiving drainage from
large areas, such as the Black Sea (cf. Degens et al.,
1976, 1991), further suggest that recent increases in
sediment input, although clearly evident, are of limited
magnitude when compared with climatically induced
changes occurring during the late Pleistocene and the
Holocene (Figure 2B).
The apparent contradictions in the information
presented above and the lack of clear evidence of substantial increases in sediment transport by the world’s
rivers as a result of human activity and land use change
could reflect the averaging associated with larger river
basins, whereby increased erosion rates may be limited to only parts of the basin. Equally, global variability in the magnitude of land use impacts will inevitably
mean that significant increases are only found in some
areas of the world. There is, however, also evidence to
suggest that the marked increases in rates of soil loss
cited above may not be reflected by increases of an
equivalent magnitude in the sediment loads of world
rivers. Alford (1992), for example, reports a study of
the 14 028 km2 Chao Phraya river basin draining the
highlands of northern Thailand which showed no evidence of a significant increase in sediment yield during
the period extending from the late 1950s to the mid
1980s, despite substantial deforestation and extensive
swidden agriculture within the basin. This apparent
discrepancy may reflect the importance of sediment
delivery processes in attenuating upstream increases in
sediment mobilisation (cf. Walling, 1983), such that
much of the additional sediment mobilised within a
river basin may be stored within the system, either on
the lower slopes and in the small tributary streams, or
on the floodplains bordering the main channel system,
and will not reach the basin outlet. There are also situations where human activity has caused a reduction,
rather than an increase, in sediment yield. More particularly, the construction of reservoirs will commonly
trap the majority of the sediment load formerly transported by the river. The closure of the Aswan Dam on
the River Nile has, for example, reduced the annual
sediment yield of that river from ca. 100×106 t year−1
to almost zero, and Meade & Parker (1985) cite the
case of the Mississippi River, U.S.A., where the construction of five major dams on the Missouri River, one
of its major tributaries, reduced the sediment yield at
its mouth by more than 50% between the 1950s and
the 1980s. In an attempt to scale up such findings to
the global scale, Vorosmarty et al. (1997) estimated
that more than 40% of the global river discharge is currently intercepted by large impoundments and that the
global sediment retention by reservoirs may exceed
226
Table 3. Increases in the sediment yields of world
rivers resulting from catchment disturbance by land
use activities, (based on data presented by Dedkov
& Mozzherin (1984))
Group
Small
basins
Large
basins
All
basins
Lowland Rivers
(n=1854)
Mountain Rivers
(n=1811)
×13.0
×8.1
×10.0
×2.2
×3.8
×2.8
25% of the global flux from the land to the oceans. It
therefore seems likely that in some areas of the globe
increases in sediment loads caused by land clearance
and land use change may be balanced by reductions
associated with reservoir development.
Changing sediment yields
The problem of assessing the magnitude of the impact of land use change and related human activity
on river sediment loads is exacerbated by the general
lack of long-term records of sediment transport by the
world’s rivers. Only a few records extend back to the
1920s and 1930s, and most cover only a few decades.
Furthermore, some sediment load records may be of
limited reliability (cf. Walling & Webb, 1981). Analysis of the available records can, however, provide
more specific evidence regarding the sensitivity of
river sediment loads to land use change, although in
many cases there will be a need to disentangle the
effects of both land use change and climate change,
which may be closely linked. Figure 3 presents information concerning long-term trends in the sediment
load of two rivers in the former Soviet Union. The
record for the 12 500 km2 basin of the Dema River at
Bochkarevo in Russia presented in Figure 3A shows
a clear trend of increasing suspended sediment yield
during the period of record which extends from 1949
to 1985. This upward trend of sediment yield is statistically significant (>95% level), but the minor increase
in annual runoff over the period of record is not statistically significant. The double mass plot of sediment
yield versus runoff suggests that the main increase in
sediment yields dates from ca. 1963 and that sediment
loads have increased by about 1.4 times since that
time. Bobrovitskaya (pers. com.) has indicated that
the increased sediment loads evidenced by this river
reflect the expansion of cultivation within the drainage basin. The second example, which is presented
in Figure 3B, relates to the Dnestr River at Sambur,
which drains an 850-km2 catchment in the Ukraine.
Here the trend line fitted to the annual sediment yields
is again highly significant statistically and suggests
that these have increased by as much as 5-fold since
the early 1950s. This increase undoubtedly reflects the
influence of forest clearance within the headwaters of
the basin (Bobrovitskaya, pers. com.), but it is also
a response to the general increase in runoff amounts
that has occurred over the period and more particularly
since the late 1960s and which also reflects climate
change. The double mass plot suggests that the impact
of forest clearance was particularly felt after 1968 and
that this itself accounts for a 1.8-fold increase in the
sediment load of the river.
Figure 4 presents examples of two other rivers,
which, in contrast to those shown in Figure 3, show
evidence of declining sediment yields. The record for
the River Isar at Munchen, Germany, which drains a
catchment area of 2855 km2 , spans the period 1930–
1990 and shows a significant reduction in sediment
yield. This reduction reflects the development of hydropower stations and associated storage reservoirs on
this river, and, more particularly, the commissioning
of the Sylvenstein Dam in 1959 (cf. Weiss, 1996).
There is no significant trend in the annual runoff totals
during the period of record, but sediment yields have
decreased to only about 20% of their former level over
this period and the trend of the double mass plot suggests that the reduction has intensified in recent years.
The other river represented on Figure 4 is the Yellow
River at Longmen, China, which drains a catchment
area of 497 561 km2 . This river is well known for its
very high sediment load and for the very high erosion
rates associated with the extensive area of thick loess
deposits which occupies much of its middle reaches.
The available record extends over 55 years and in this
case shows evidence of a significant decline in both
runoff and sediment yield during this period. Sediment
yields can be seen to reduce by about 50% during the
period of record and much of this reduction can be
attributed to the extensive soil and water conservation
works undertaken in the middle reaches of the Yellow
River basin since the 1970s, which have reduced both
runoff and sediment mobilisation. The reduction in
runoff over this period also reflects a shift to a drier
climate and reduced annual rainfall totals. Attempts by
Chinese workers to assess the relative importance of
soil and water conservation works and climate change
227
Figure 3. Trends in suspended sediment yield and runoff for the Dema River at Bochkarevo, Russia, 1949–1985 (A) and the Dnestr River at
Sambur, Ukraine, 1950–1983 (B). (Based on data supplied by Professor Bobrovitskaya, State Hydrological Institute, St Petersburg, Russia.)
in accounting for the reduced runoff and sediment
loads in the main Yellow River have suggested that
the former accounts for ca. 60% of the reduction in
runoff and ca. 50% of the reduction in sediment yield
(cf. Gu, 1994; Mou, 1996). The double mass plot
further emphasises that the reduced sediment load is
not simply the result of reduced runoff, since, if the
relationship between annual sediment load and annual
runoff can be assumed to be linear, no change in the
slope of the double mass plot would be expected if
the reduction in sediment yield was solely a reflection
of reduced runoff. In this case, the double mass plot
suggests that a reduction in sediment yield of approximately 40% has occurred since the late 1970s, which
cannot be directly linked to the reduction in runoff during this period. The potential significance of soil and
water conservation measures in reducing the sediment
loads of large river basins is further emphasized by the
records for the tributaries of the Yellow River which
drain solely areas within the loess region. In the case of
the Wuding River (29 662 km2 ), sediment yields have
declined by almost 90% between the 1950s and the
early 1990s (Figure 5), although part of this decline is
again due to climate change and the associated reduction in precipitation. The double mass plot presented
in Figure 5 suggests that a reduction in sediment yield
of ca. 60% has occurred since 1971, which cannot be
directly linked to the reduced runoff.
As a final example of the sensitivity of river sediment loads to land use change, Figure 6 presents data
for two rivers which show little evidence of changing sediment loads over a periods of more than 40
years. The record of annual sediment loads for the
River Lech above Fussen in Bavaria (1422 km2 ) extends over more than 60 years and, since this river has
been essentially unaffected by impoundment and regulation, it affords a valuable example of the response
of an Alpine river to changing land use. The annual
runoff record shows evidence of a small reduction
over the period of record and this trend is statistically
significant at the 95% level, but the sediment record
shows little evidence of change, despite the likelihood
of significant changes in both land use and land management practices within the catchment over the past
228
Figure 4. Trends in suspended sediment yield and runoff for the River Isar at Munich, Bavaria, 1930–1990 (A) and the Yellow River at
Longmen, China, 1935–1989 (B). (Based on data supplied by Dr F.H. Weiss, Bayer Landesamt fur Wasserwirtschaft, Munich, and Dr Fang
Xuemin, IWHR, Beijing.)
60 years (cf. Summer et al., 1996). A similar picture is
provided by the annual runoff and sediment load data
for the Upper Yangtze River at Yichang, China (Figure 5B). This river drains an area of 1 005 501 km2 ,
which supports a population of ca. 140 million. The
population has increased rapidly from ca. 60 million
in 1953 and there are many reports of increased soil
loss (cf. Dai & Tan, 1996). Despite such evidence
of increased population pressure and intensification
of land use, the time series of annual sediment loads
is essentially stationary and there is no evidence of
decreasing runoff, which might offset the impact of
increased erosion. Some evidence of increased sediment loads is provided by the shorter records for some
of the tributary rivers within the Upper Yangtze basin
(cf. Lu & Higgitt, 1998), but again this suggests that
sediment loads have exhibited only limited change.
The results presented above afford coverage of
only a small number of rivers and a limited range of
physiographic conditions. Nevertheless, they provide
evidence of the complexity of the causal link between
land use change and associated increases in rates of
erosion and river sediment loads. The records from
the two rivers in the former Soviet Union depicted in
Figure 3 show significant increases in sediment yields
consequent upon land use change and land disturbance, and the records from the Yellow River presented
in Figure 4B and Figure 5 similarly demonstrate the
229
Figure 5. Trends in suspended sediment yield and runoff for the
Wuding River, China, 1957–1993. (Based on data supplied by Mr
Mou Jinze, YRCC, Zhengzhou.)
sensitivity of the sediment yields from this river basin
to reduction by means of soil and water conservation
measures. Conversely, however, the records from the
River Lech and the Upper Yangtze presented in Figure 6 provide examples of situations where sediment
loads appear to be essentially insensitive to changes
occurring within the catchment. These apparently conflicting findings reflect, at least in part, variations in
what might be termed the ‘buffering capacity’ of the
catchments involved. The work of Meade & Trimble
(1974) and Phillips (1992) in assessing the impact
of post-colonial agricultural activity on the sediment
loads of rivers draining to the coastal seaboard of the
eastern United States has usefully demonstrated how
much of the sediment eroded from their watersheds
was deposited or stored within the river basin and
failed to reach the basin outlet. Subsequent reductions
in soil erosion associated with the decline of plantation
agriculture and the introduction of soil conservation
measures were also not reflected by downstream sediment loads, since reductions in sediment transport
were balanced by remobilisation of stored sediment.
Major changes in land use which would have been accompanied by changing erosion rates were therefore
not reflected by major changes in sediment transport
within the lower reaches of these rivers. The effectiveness of such buffering can be related to the classic
sediment delivery ratio concept (cf. Walling, 1983).
Rivers with a low sediment delivery ratio will be characterized by a high degree of buffering, since much of
the sediment mobilized within the catchment will be
deposited and these deposits can in turn be remobilized if sediment supply to the river system declines.
Where, however, sediment delivery ratios are high,
little sediment will be deposited and there will be
only limited sediment available to be remobilized during times of reduced supply. Such river basins can
therefore be expected to be poorly buffered and the
sediment output will be sensitive to changing erosion
rates associated with changing land use. Estimates of
sediment delivery ratios available for the Middle Yellow River basin (cf. Gong & Xiong, 1980; Mou &
Meng, 1980) indicate that much of this region is characterised by values close to 1.0 or 100%, due in part to
the high drainage density and the frequent occurrence
of hyperconcentrated flows, which reduce the potential
for sediment deposition. These values are consistent
with the low degree of buffering found in this river
basin. Values of the sediment delivery ratio cited for
the Upper Yangtze River basin by Dai & Tan (1996)
and Liu & Zhang (1996) are much lower than those
for the Middle Yellow River basin and averaged ca.
0.34 or 34%. Such values are likewise consistent with
the much higher degree of buffering suggested by the
record of sediment loads for the Upper Yangtze (cf.
Figure 6B). No corresponding information on sedi-
230
Table 4. The suite of sediment properties used to
fingerprint suspended sediment sources and the
results of the Multivariate Discriminant Function
analysis used to select this composite fingerprint
Tracer property
Cumulative % samples
classified correctly
N
Total P
Sr
Ni
Zn
226 Ra
137 Cs
unsupported 210 Pb
Fe
Al
51.9
74.3
77.9
82.4
83.8
83.8
88.2
88.2
92.7
94.1
Based on Walling et al. (1999a).
ment delivery ratios are available for the other rivers
documented in Figures 3–6, although existing evidence would suggest that SDR values for the River Lech
would be relatively low and thus indicative of a wellbuffered system, as suggested by the stability of the
long-term record of sediment loads.
Sediment budgets
The above discussion emphasizes that any attempt
to consider the impact of land use on erosion and
sediment transfer through a river basin should direct attention not only to changes in sediment load
at the catchment outlet, but also to the entire delivery or conveyance system and to the sinks or stores
involved. This broader perspective will permit improved understanding of the link between land use,
erosion and sediment yield as well as focusing attention on the fate of mobilised sediment. Although
the detrimental effects of increased sediment loads in
rivers are well documented (cf. Clark et al., 1985),
accumulation of sediment in sinks and stores within
a drainage basin is known to give rise to other environmental problems. The sediment budget concept
(cf. Dietrich & Dunne, 1978; Walling, 1988) affords
a convenient means of representing and interpreting
this link and Figure 7, based on the classic work of
Trimble (1981), provides a useful example of its application. In this case, data from a range of field and
documentary sources were used to reconstruct the sediment budget of Coon Creek, a 360-km2 basin in the
Driftless Area of Wisconsin, U.S.A., for two periods in the past, namely 1853–1938 and 1938–1975.
The first period represented one of poor land management which resulted in severe soil erosion, whereas
the second was characterised by the introduction of
conservation measures. During the first period, large
volumes of soil were eroded from the slopes of the
basin, but only a small proportion (ca. 5%) of this
was transported out of the basin. Most of this material was stored within the catchment. During the latter
period when widespread soil conservation measures
were introduced, rates of soil loss from upland sheet
and rill erosion were reduced by about 25%, but sediment yields at the basin outlet remained essentially
the same, because sediment stored in the tributary valleys and upper main valley was remobilized. In this
case, therefore, substantial changes in land use and
land management within the upstream basin were not
reflected in the downstream sediment yield because of
the low SDR of the basin and thus its relatively high
buffering capacity.
The precise form of the sediment budget of a catchment, and thus its SDR and buffering capacity, will
vary according to the local physiographic conditions,
but considerable variation may also occur even within
a relatively small area. Figure 8, for example, illustrates the very considerable diversity of sediment
budget structure reported by Golosov et al. (1992) for
small- and medium-sized drainage basins on the Russian Plain, a region which is heavily impacted by land
use activities and soil erosion. In these examples, the
sediment delivery ratios ranged from zero to 89%. In
the former case, changes in land use and erosion rates
would not be reflected in changes in sediment yield,
whereas in the latter case sediment yields could be
expected to be highly sensitive to such changes.
Scaling up sediment budgets such as those shown
in Figures 7 and 8 to much larger heterogeneous river
basins clearly involves many problems in terms of assembling the information necessary to establish the
key elements of the sediment budget and such studies have hitherto been constrained by the limitations
of traditional monitoring techniques for quantifying
sediment budgets. As a result much of our existing understanding of the sediment budgets of larger
river basins is based on theory and inference rather
than direct empirical evidence. However, recent work
has demonstrated the potential of newly developed
techniques for establishing the relative importance of
different sediment sources within a drainage basin
and for assessing the importance of overbank flood-
231
Figure 6. Trends in suspended sediment yield and ruonff for the River Lech at Fussen, Bavaria, 1924–1990 (A) and the Yangtze River at
Yichang, China, 1950–1991 (B). (Based on data supplied by Dr F. H. Weiss, Bayer Landesamt fur Wasserwirtschaft, Munich and Dr Fang
Xuemin, IWHR, Beijing.)
plain deposition and associated floodplain storage as
a component of the sediment budget. This potential
can usefully be demonstrated by referring to the results of some recent work undertaken by the author
and his co-workers in the catchment of the River Ouse
in Yorkshire within the framework of the U.K. Land–
Ocean Interaction Study (LOIS). This work was aimed
at elucidating the key features of the sediment budget
of this 4000-km2 catchment and involved establishing the relative importance of the primary sediment
sources and quantifying the role of floodplain sedi-
mentation as a sediment sink (cf. Walling et al., 1998,
1999a,b).
The fingerprinting approach, involving multicomponent fingerprints (cf. Walling et al., 1993; Walling
& Woodward, 1995), was used in the Ouse catchment
to determine the relative importance of four potential
sediment sources within the catchment, namely, the
surface of cultivated areas, areas of permanent pasture and upland moorland, and areas of woodland,
and channel banks and other sources of subsurface
material. In essence, the fingerprinting approach in-
232
Table 5. Load-weighted contribution of the major source types to suspended sediment samples collected from the Rivers Swale, Ure, Nidd, Ouse and Wharfe during
the period November 1994 to February 1997
River
Number of
suspended
sediment
samples
Swale
Ure
Nidd
Ouse
Wharfe
19
14
14
30
7
Woodland
topsoil
(%)
0
0.7
6.9
0
4.4
Uncultivated
topsoil
(%)
Cultivated
topsoil
(%)
Channel
bank
material
(%)
41.8
45.1
75.2
24.6
69.5
30.0
17.0
2.8
38.1
3.6
28.2
37.2
15.1
37.3
22.5
Based on Walling et al. (1999a).
Figure 8. The sediment budgets of four drainage basins on the
Russian Plain, as documented by Golosov et al. (1992).
Figure 7. The sediment budgets of Coon Creek, Wisconsin, U.S.A.,
reconstructed by Trimble (1983) for the periods 1853–1938 and
1938–1975.
volves comparison of the geochemical properties of
suspended sediment transported by the river with those
of potential sources, in order to establish their relative importance. Use of statistically verified composite
fingerprinting signatures involving several different
sediment properties (cf. Table 4), enabled the four
potential sources to be clearly discriminated and a
multivariate mixing model was used to determine the
relative contribution of the four sources to suspended sediment samples collected from the River Ouse
and its major upstream tributaries and from the River
Wharfe, which joins the Ouse below its tidal limit (cf.
Figure 9). The load-weighted estimate values for the
relative contributions from the four different sediment
sources are listed in Table 5. These results confirm that
a major proportion of the suspended sediment load
of the River Ouse and its tributaries is derived from
surface sources and is thus likely to be directly influenced by land use activities, including cultivation and
grazing pressure.
In upscaling from smaller to larger catchments, it is
important to recognize that the river floodplains which
border the main channel systems in larger river basins
can represent an increasingly important sediment sink,
which will attenuate changes in upstream sediment
inputs. Walling & Quine (1993), for example, estimated that floodplain storage accounted for ca. 23% of
233
Table 6. A comparison of the estimates of total storage of sediment on the floodplains bordering the main channel system of the River Ouse
and its primary tributaries, with the estimated suspended sediment loads for the study rivers
River
Floodplain
storage
(t year−1 )
Mean annual
suspended
sediment load
(t year−1 )
Total sediment
derived to
channel
(t year−1 )
Floodplain
storage as %
of sediment
input
Swale
Nidd
Urea
Ousea
19214
7573
15125
18733
42352
7719
28887
61566
15292
44012
31.2
49.5
34.3
Total to Ouse
gauging station
49041
75111
124152
39.5
Total to tidal limit
60645
Wharfe
10325
10816
21141
48.8
a Ure is the River Ure to its confluence with the River Swale, while Ouse refers to River Ure/Ouse from below this point to the tidal limit.
Based on Walling et al. (1998).
the sediment delivered to the main channel system of
the 6850-km2 catchment of the River Severn in the
U.K. Attempts to document rates of overbank sedimentation on river floodplains, in order to establish
the importance of such conveyance losses, face many
practical problems. However, recent developments in
the application of the environmental radionuclide Cs137 to estimate rates of overbank sedimentation offer
considerable potential in this context (cf. Walling &
He, 1997a,b). This approach was used by Walling
et al. (1998) to establish the significance of floodplain storage in the suspended sediment budget of the
main channel systems of the Rivers Ouse (3315 km2 )
and Wharfe (818 km2 ). In this case, more than 250
sediment cores were collected from 26 representative transects located along the main channel systems
of the Yorkshire Ouse and its major tributaries and
the River Wharfe (cf. Figure 9) for Cs-137 analysis.
The estimates of mean sedimentation rates for the individual transects obtained from these cores, which
averaged ca. 0.2 g cm−2 year−1 , were extrapolated to
the individual reaches between adjacent transects and
the mean annual conveyance loss associated with overbank sedimentation on the floodplain bordering the
main channel system was calculated. By relating these
losses to the mean annual sediment loads of the rivers
(Table 6), it was possible to establish the relative importance of floodplain storage in the sediment budget
of the main channel system (Table 6, Figure 10). In
the case of the main River Ouse system, floodplain de-
position accounted for ca. 40% of the total amount of
suspended sediment delivered to the main channel system over the past 40 years and for the River Wharfe the
equivalent value was ca. 49%. These conveyance loss
values of 40% or more, associated with the floodplains
bordering the main channel system of a 4000-km2
river basin, serve to emphasize further the potential
importance of storage in attenuating the link between
upstream erosion and downstream sediment loads and
thus the impact of land use change on sediment yield.
The application of environmental radionuclides
in quantifying the role of river floodplains as sediment sinks also offers the possibility of investigating
changes in sedimentation rates through time. In this
way, studies of changing sediment output from a
drainage basin could be paralleled by studies of changing sediment storage. By measuring both the Cs-137
and unsupported Pb-210, content of floodplain cores,
it is possible to derive estimates of sedimentation
rates over the past 35–45 years (Cs-137) and 100–120
years (unsupported Pb-210) which can be compared
(cf. Walling & He, 1994, 1999). The basis of these
estimates precludes separation of the record into consecutive periods, but, by comparing the longer-term
with the shorter-term estimate, it is possible to establish whether sedimentation rates have increased or
decreased in recent years. Table 7, which is based
on the work of Walling & He (1999), identifies the
trends evidenced by single cores collected from the
floodplains of a representative selection of 21 rivers
234
Figure 9. The catchments of the River Ouse and Wharfe, Yorkshire, U.K.
in the U.K. The results show no evidence of major
changes in sedimentation rates over the past 100 years,
but the individual rivers provide examples of situations
where sedimentation rates have both increased and
decreased and remained effectively stable. Increases
and decreases have been defined as instances where
the sedimentation rate for the past 33 years has increased or decreased by more than 10% relative to
the sedimentation rate for the past 100 years. This
period was marked by significant changes in land use
and land management practices, but it appears that
such changes had relatively little impact on rates of
floodplain sedimentation and storage. In these circumstances, it is likely that sediment outputs from the
river basins represented also changed relatively little.
Thus, although floodplain storage could represent an
important buffer for changing sediment fluxes, such
effects would appear to have been of limited import-
235
Table 7. A comparison of mean annual sedimentation rates for the past 33 and 100 years estimated for a selection of
sites on the floodplains of British rivers
River/Location
Sedimentation rate (g cm−2 year−1 )
past 33 years
past 100 years
Trenda
1. River Ouse near York
2. River Vyrnwy near Llanymynech
3. River Severn near Atcham
4. River Wye near Preston on Wye
5. River Severn near Tewkesbury
6. Warwickshire Avon near Pershore
7. River Usk near Usk
8. Bristol Avon near Langley Burrell
9. River Thames near Dorchester
10. River Torridge near Great Torrington
11. River Taw near Barnstaple
12. River Tone near Bradford on Tone
13. River Exe near Stoke Canon
14. River Culm near Silverton
15. River Axe near Colyton
16. Dorset Stour near Spetisbury
17. River Rother near Fittleworth
18. River Arun near Billingshurst
19. River Adur near Partridge Green
20. River Medway near Penshurst
21. River Start near Slapton
0.95
0.21
1.22
0.15
0.86
0.46
0.88
0.39
0.51
0.70
0.60
0.56
0.45
0.35
0.51
0.04
0.11
0.39
0.51
0.15
0.51
Stable
Decrease
Decrease
Decrease
Stable
Decrease
Decrease
Increase
Decrease
Decrease
Stable
Increase
Stable
Stable
Increase
Stable
Decrease
Decrease
Decrease
Decrease
Increase
1.04
0.46
1.42
0.28
0.95
0.66
1.01
0.33
0.64
0.93
0.65
0.43
0.42
0.32
0.40
0.04
0.14
0.48
0.71
0.23
0.45
a The trend indicates the change in sedimentation rate when comparing the past 33 years with the past 100 years. A
change of >±10% is taken to represent a significant increase or decrease.
Based on Walling & He (1999).
ance in these river basins. It is, nevertheless, possible
that significant changes in erosion rates were buffered
by loose coupling of the slope and channel systems
and sediment storage in headwater areas, since such
sinks are likely to be of similar importance to floodplain storage within the lower reaches of the main
river system. Further work is clearly required to elucidate the behaviour of this other potentially important
component of the catchment sediment budget.
Sediment storage
Figure 10. The role of overbank floodplain sedimentation in the
sediment budget of the main channel systems of the River Ouse and
its major tributaries and the River Wharfe, Yorkshire, U.K.
Recognition of the potential importance of sediment
storage within a drainage basin and its role in attenuating the impact of land use change on downstream sediment loads also directs attention to the wider environmental significance of such storage. Deposition of sediment and, more importantly, sediment-associated nutrients and contaminants in sediment sinks could constitute a significant environmental problem in terms
of both their accumulation and storage and the po-
236
tential for subsequent remobilisation, such that the
storage response of a river basin to land use change
could be of equal importance to changes in sediment
output. Marron (1987), for example, cites the case
of the Belle Fourche River in South Dakota, U.S.A.,
where gold mining within the upstream catchment
during the period extending from the 1880s to 1977
caused the accumulation of large quantities of arseniccontaminated sediment within the river floodplain and
it was estimated that, even with no further inputs from
mining activity, arsenic contamination would remain a
problem within this river system for many centuries to
come due to remobilisation of contaminated sediment.
Recent concern for phosphorus cycling within terrestrial and freshwater ecosystems has also directed
attention to the potential role of river floodplains in
storing sediment associated-P mobilized from the upstream catchment. Since particulate-P can account for
a major proportion of the total-P load of a river, overbank sedimentation on floodplains can result in substantial conveyance losses of P and appreciable quantities
of P can accumulate in such floodplain sinks. Figure 11 provides information on the P content of a sediment core collected from the floodplain of the River
Culm in Devon U.K. In this case, total-P concentrations in the upper horizons of the floodplain sediment,
which average ca. 2600 mg kg−1 are more than twice
those associated with soils in adjacent areas above the
level of the floodplain.. These higher concentrations
reflect the deposition of sediment-associated P mobilised from the upstream watershed P in association with
fine sediment. Selective erosion, involving preferential mobilisation of fines and organic matter, causes
enrichment of the P content of suspended sediment
relative to the bulk soil, and this enrichment is in turn
reflected in the high concentrations found in the floodplain sediments. In addition to being characterized by
higher total-P concentrations, the floodplain sediments
also possess a much greater P stock and this stock
will increase through time as deposition continues.
Figure 11 also presents information on the downcore
variation of Cs-137 concentrations in the floodplain
core and this information can be used to estimate the
rate of sediment deposition, because the level associated with the peak Cs-137 concentration can be used to
establish the floodplain surface at the time of peak fallout rates which occurred in 1963 (cf. He & Walling,
1996). In this case the deposition rate is estimated to
be 0.31 g cm−2 year−1 and, if this value is combined
with the average total P content of sediment in the upper part of the core (i.e. post 1963), an annual input of
sediment-associated P to the floodplain surface of 8.2
g m−2 year−1 can be estimated. Equivalent values for
the total P concentration of post 1963 floodplain sediment deposits and the P accumulation rate have been
obtained for sediment cores collected from 19 further
sites on the floodplains of U.K. rivers and these values
are presented in Table 8. The range of P concentrations
associated with floodplain sediments listed in Table 8
closely reflects the intensity of agricultural activity in
the upstream catchment, although point-source inputs
of P to the river system from sewage works will clearly
also exert a significant influence. The values of P accumulation presented in Table 8 are of a similar order
of magnitude to that of 9 g m−2 year−1 reported by
Fustec et al. (1995) for a floodplain site on the River
Seine at Maizières, France, and further underscore the
potential importance of river floodplains as a phosphorus sink. Remobilisation of these sediment stores
could release large amounts of P back into the rivers,
although rates of bank erosion associated with U.K.
rivers are such that this sediment is likely to have a
residence time of 102 to 104 years and can be viewed
as being in long-term storage. Major changes in channel mobility and floodplain erosion, such as might
accompany climate change, must, however, be seen
as a potential cause of more rapid remobilisation.
Consideration of downcore changes in the P content of floodplain sediment also affords a means of
deriving some information on past changes in the P
content of suspended sediment transported by the river
concerned. In this case it is necessary to recognize the
potential for post-depositional changes in P content,
due to mobilisation into solution, plant uptake etc. but,
since most of the sediment-associated P is likely to be
firmly fixed to the sediment and not readily mobilised,
downcore changes in P concentration will provide
some tentative evidence concerning past changes in
the P content of deposited sediment. By coupling this
information with information on average sedimentation rates derived from the Cs-137 profile, it is possible
to establish the likely trend of the P content of deposited sediment over the past 40 years or more. Such
information is presented in Figure 12 for a representative selection of five U.K. rivers. In this case there is
close relationship between both the P concentrations
involved and the trend of these concentrations over
the period since 1950, and the intensity of agricultural
activity in the upstream catchment. The catchments of
both the Usk and the Upper Severn are characterized
by low intensity agriculture with substantial areas of
upland sheep pasture and the sediment collected from
237
Figure 11. Down-core variations in total phosphorus (A) and caesium-137 (B) for a sediment core collected from the floodplain of the River
Culm near Columbjohn, Devon, U.K.
Figure 12. Variations in the total phosphorus content of sediment deposited on the floodplains of several British rivers during the period
1950–1990.
the floodplains of these two rivers exhibits relatively
low P concentrations with only limited evidence of
an upward trend. This is consistent with the relatively
low levels of fertiliser application in these two catchments. In contrast, the data obtained for the Rivers
Axe, Arun and Torridge show both higher P concentrations and a more well-defined upward trend, which are
consistent with the more intensive agriculture in their
catchments. The catchment of the River Torridge is
occupied primarily by permanent pasture, whereas the
catchments of the Axe and Arun are characterized by
areas of both both pasture and arable cultivation. In the
case of the River Torridge, the P content of sediment
deposited on the floodplain has increased by nearly
50% over the past 40 years, whereas the increases for
the Arun and Axe are ca. 94 and 170%, respectively.
Equivalent values for 15 other sites, for which information on the P content of floodplain sediment and P
accumulation rates were provided in Table 8, are also
are presented in Table 8. This larger data set shows further evidence of substantial increases in the P content
of sediment deposited in the floodplain store of British
rivers over the past 40 years, which are likely to reflect
land use change and increased fertilizer application
within the upstream catchments. Recognition of the
significance of sediment as a carrier for nutrients and
contaminants clearly introduces a need to take account
of both the amount and the properties of sediment
238
Table 8. Phosphorus storage on the floodplains of British Rivers
River/Location
Mean Total P
concentration of
post 1963 sediment
(mg kg−1 )
Total P
storage
since 1963
(g m−2 year−1 )
1. River Vyrnwy near Llanymynech
2. River Severn near Atcham
3. River Wye near Preston on Wye
4. River Usk near Usk
5. River Teme near Broadwas
6. River Torridge near Great Torrington
7. River Taw near Barnstaple
8. River Exe near Stoke Canon
9. River Culm near Silverton
10. River Start near Slapton
11. River Tone near Bradford on Tone
12. Bristol Avon near Langley Burrell
13. River Thames near Dorchester
14. Warwickshire Avon near Pershore
15. River Axe near Colyton
16. Dorset Stour near Shillingstone
17. River Rother near Fittleworth
18. River Arun near Billingshurst
19. River Adur near Partridge Green
20. River Medway near Penshurst
760
717
1031
417
507
955
830
1090
2660
1931
1047
1266
1532
2374
1466
1321
1895
1436
795
913
1.79
9.0
1.6
3.8
2.0
6.5
4.8
4.8
8.2
8.4
5.8
5.0
7.8
10.6
7.6
11.6
2.0
5.7
4.0
1.3
stored within the delivery system and the influence of
land use on both aspects.
Perspective
This contribution has endeavoured to review existing
understanding of the relationship between land use
change, erosion and sediment yield. Although the impact of land use change on rates of soil loss, and more
particularly the impact of land clearance and cultivation in increasing erosion rates, has been extensively
documented, the evidence for major changes in the
sediment loads of larger rivers is less clear. Existing
long-term records provide evidence of cases where
sediment loads have both increased and decreased as
a result of land use change and other human activity,
but it is clear that river basins and more particularly
river systems possess considerable capacity to buffer
such changes. The extent of this buffering capacity
can be expected to be closely related to the sediment
delivery ratio of the basin, such that river basins with
Increase in
Total P content
of deposited
sediment
1950–1992 (%)
23
17
25
45
45
75
65
33
40
55
28
30
23
170
53
33
94
147
10
low sediment delivery ratios will be characterized by a
high buffering capacity and vice versa. Recognition of
the importance of sediment storage within a drainage
basin necessarily means that attempts to understand
the linkages between land use, erosion and sediment
yield should consider the overall sediment budget and
the associated sources and sinks, rather than only the
sediment outputs. Changes in the functioning of these
stores could be of greater significance than changes in
sediment output from a drainage basin. Recent work
has demonstrated an improved capacity for assembling the information needed to construct a sediment
budget and investigations of the role of floodplain
sedimentation as a sediment sink have increasingly
emphasised the potential importance of this store. By
directing attention to the overall sediment budget of a
river basin and to the associated sediment stores and
sinks, as well as the sediment output, the wider environmental significance of these components of the
sediment budget becomes increasingly apparent. They
can represent important sinks for sediment-associated
contaminants and any attempt to fully understand the
239
cycling and transfer of phosphorus within a river
basin must, for example, take account of the potential
importance of river floodplains as P sinks.
Acknowledgements
The work reported in this paper draws both on data
generously provided by others and on the results of
several investigations undertaken by the author and his
co-workers in recent years. These inputs are gratefully
acknowledged. Particular thanks are extended to Professor N. Bobrovitskaya, Dr F.H. Weiss, Mr Mou Jinze
and Dr Fang Xuemin for their help in providing the
long-term sediment yield data presented in Figures 3–
6, to Dr Qingping He and Dr Phil Owens for their
valuable collaboration over the past 5 years, to Art
Ames and Will Blake for their help with P analysis
and to Mr Terry Bacon for producing the diagrams.
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