INVESTIGATING SEDIMENT DELIVERY IN LARGE RIVER
BASINS
DAVID L. HIGGITT and LU XIXI
Department of Geography, National University of Singapore,
1 Arts Link, Singapore 117570.
The sediment yield of rivers is often sensitive to changes in catchment land use. Such an
impact is most likely to be demonstrated at a restricted spatial scale, particularly in
headwater catchments. In larger river basins the link between land use change and
sediment yield is less distinct as sediment delivery processes moderate overall catchment
response. This has been illustrated by the well known geomorphological work of S.W.
Trimble in the upper Mississippi catchment where sediment yields to the main river have
experienced limited change despite major switches in the predominant sources. In turn
this prompts questions about the extent to which long term trends in sediment flux from
large basins mask information about changing erosion and sediment transport dynamics
from constituent parts of the river basin. Recent work in the Upper Yangtze catchment,
China, has attempted to analyse the spatial and temporal controls on sediment yield
which has implications for the Three Gorges Project. Here it is apparent that despite
considerable evidence for deforestation and enhanced soil erosion in recent years,
sediment yields at the catchment outlet have not exhibited an upward trend. The paper
introduces three approaches for examining this paradox. The first is based upon a
distributed multivariate analysis of within-catchment sediment yields and attempts to
incorporate land cover information. The second involves responsive field investigation at
critical locations within the basin, in the form of rapid appraisal. The third involves the
modelling of the impact of progressive catchment disturbance on net sediment yield time
series. Simulation models based on a prediction of erosion rate coupled to an operational
function for sediment delivery can be modified to incorporate rules for connectivity,
network distribution, source types and storage. Analysis demonstrates that prediction of
longer-term sediment yield in large catchments, which may be critical to water resource
management, are sensitive to many aspects of sediment delivery.
INTRODUCTION
Identifying the controls on recent changes in soil erosion, sediment delivery and sediment
yield is an issue of concern to policy makers engaged in the management of rivers with
high sediment loads. At the same time, the major biogeochemical role of rivers in
transporting nutrients from continents to oceans has renewed interest in fluvial sediment
dynamics. In this respect the sediment fluxes of large rivers are of particular
significance. It has been estimated (Chen, [1]) that 40% of freshwater and particulate
matter entering the oceans is derived from the ten largest rivers. Development projects
that seek to harness water resources in large rivers may, on the one hand, require
management strategies to circumvent negative impact form high sediment fluxes, while,
on the other hand, they may substantially impact on the water and sediment flux reaching
the ocean with consequences for coastal fisheries. Although there are several examples
of accelerated erosion and increased sediment loads associated with anthropogenic
activity, such as deforestation, vegetation clearance or land use change, some human
activity may not lead to obvious trends, particularly in large catchments (Walling, [2]).
This has been illustrated in the upper Mississippi catchment where sediment yields to the
main river have experienced limited change despite major switches in the predominant
sources (Trimble, [3]). An example of this situation can also be found in the Upper
Yangtze river, China. Several authors have noted that there is no systematic trend in
sediment loads in the Upper Yangtze (Gu et al., [4]; Gu and Douglas, [5]; Dai and Tan,
[6]), despite evidence that soil erosion has increased markedly within the catchment. The
dynamics of erosion and sediment transport is of concern in this river because of its
potential impact on the Three Gorges Dam.
The paper explores three ways in which the sediment delivery dynamics of the
Yangtze have been investigated. The first section describes a procedure for undertaking
multivariate and time series analysis using all stations within a catchment. The second
zooms in to investigate the sediment dynamics of a major tributary where sediment yield
has trended upwards in recent years. The third introduces some simple modelling
routines that explore the relationship between the spatial signature of disturbance and
yield at the catchment outlet.
SEDIMENT DELIVERY IN LARGE CATCHMENTS
While many studies in geomorphology have attempted to explain global variation in
fluvial sediment yields, until recently few have investigated sediment delivery
characteristics within large basins. Whereas global and regional scale surveys tend to
concentrate on sediment load data from the outlet gauging station, investigation of spatial
and temporal variability within the catchment requires data to be derived from many
gauging stations. In a few cases, such as the large rivers of China, there are several
gauging stations which have operated and summary data were available in the public
domain until 1987. Pooling the data into a usable format is a major undertaking but once
constructed as a database it can be scrutinised to examine patterns of sediment delivery
and changes over time.
A multivariate analysis of the Upper Yangtze sediment yield data was attempted
(Higgitt and Lu, [7]). The procedure derives a series of variables describing topography,
climate, hydrology, population density and land cover for the sub-catchment area above
each of 62 gauging stations with long term records. The multiple regression explains
much of the variability in sediment yield in the western part of the catchment, but
performs less well in the densely populated eastern half where there are more water
conservation structures and a more complicated mosaic of land uses. The full data set
(256 gauging stations) was used to explore time series. There are 16 stations that
displayed a significant trend over time (Figure 1a). Breaking the times series into
decadal components it is possible to plot the sub-catchment areas that experienced
increasing sediment yields in the last decade of public domain records (up to 1987).
These area are in the Three Gorges region itself, the Wu tributary and the Dadu tributary
(Figure 1b). Each of these areas was reported to be experiencing deforestation and
agricultural expansion during this time period.
RAPID APPRAISAL OF CRITICAL AREAS
Field investigation of sediment-related problems tends to be restricted to relatively small
basins or to specific water resource issues. A catchment area of over 1 million km 2
makes fieldwork impractical, yet an over-reliance on GIS may lead to spurious results.
For this reason, the preceding analysis that demonstrated the increasing trends in
sediment yield prompted a field visit to undertake a rapid appraisal of soil erosion status.
The rapid appraisal of sediment budgets has been ably documented by Reid and
Dunne [8]. In contrast to detailed research programmes the aim of rapid appraisal is to
affirm the main sources of erosion such that some tangible strategies can be suggested for
land use management. This does not require precise measurements and prompts the use
of field mapping and analysis of satellite imagery or aerial photography.
The Dadu River is a right bank tributary of the Min River which in turn drains into
the Yangtze. The mountainous area of western Sichuan through which the Dadu flows
attracted agricultural expansion throughout the 1980s and 1990s.
There have
subsequently been attempts to afforest the slopes and much agricultural land has been
abandoned. Numerous gully systems break through the agricultural terraces on the slopes
of the main Dadu Valley. A stretch of just under 200 km of river valley was mapped
starting 20 km north of Luding and finishing at Hai Yan. The number of erosion features
(gully systems and small landslide complexes) and the number of major landslides were
recorded for each 5 km reach of valley. This represents number of features per 10 km of
valley side. The land use for each reach was classified into three categories: agriculture
extending more than halfway up slope; agriculture on lower slopes with abandoned
terraces above; and mainly forested. The frequency of erosion features caries markedly
in the downstream direction (Figure 2a). When these are averaged across land use types
the erosion features per 5 km reach for each land use category is 10.9, 7.6 and 5.1
respectively.
Figure 1. Locations within the Upper Yangtze catchment experiencing significant
changes in sediment yield. A) Location of gauging stations; B) Sub-catchments with
significant changes in sediment yield for period 1977-87. Solid shading = decrease in 0250 t km-2 year-1 range; Horizontal bars = increase in 0-250 t km-2 year-1 range; Crosshatch = increase in 250-500 t km-2 year-1 range (after Lu and Higgitt, [9]).
The frequency of major landslides is 3.5, 1.5 and 0.6 respectively. A similar pattern
emerges in the Lu Sa He tributary. Though based on simple measures, the mapping
illustrates that agricultural expansion is the main trigger in increasing sediment delivery
from the slopes of the main valley. Analysis of satellite images (Chen et al., [10])
indicates that land cover change is marked along the main valley but limited elsewhere
(Figure 2b). Some of the larger landslides can be mapped directly from ETM+ imagery.
Thus, a rapid appraisal procedure involving simple field mapping and analysis of
satellite imagery confirms that the observed increase in sediment yield in the Dadu is
related to expansion of agriculture on the main valley slopes.
Number of erosion features
30
25
20
15
10
5
95
11
0
12
5
14
0
15
5
17
0
18
5
80
65
50
35
5
20
0
Distance dow nstream (km )
Figure 2. Distribution of erosion features along the Dadu Valley. A) Frequency
distribution of erosion feature mapped in the downstream direction for 190 km. Major
landslides are indicated by upper pale bars with other erosion features (large gully
systems and smaller landslides below); B) Land cover classification of the upper part of
the mapped reach derived from ETM+ imagery, April 2001 (from Chen et al., [10]). The
swath of disturbance along the main valley has dramatically increased in area in the last
two decades.
MODELLING IMPACTS OF DISTURBANCE
The field evidence supports a link with land cover transformation (in particular
deforestation and extension of agriculture onto steep slopes) that has been registered in
the sediment yield time series from a major Yangtze tributary. However, the sediment
yield at the site of the Three Gorges Dam has not shown any marked tendency despite
many observations of increasing soil erosion throughout the basin. This may in part
reflect the impact of large dams on the tributaries that serve to trap sediments moving
towards the main channel. By examining time series of sediment yields from gauging
stations upstream and downstream of dam construction sites it is clear that the increasing
trends above the site diverge from strongly decreasing sediment yields below. One
example is the Bikou Dam on the Jialing tributary that was completed in 1975. The Tuo,
Fu and Qu rivers that drain the southern part of the Sichuan Basin have high ratios of
reservoir capacity relative to catchment area and a decrease in sediment yield on the Tuo
might be attributed to high aggregate reservoir capacity introduced to its catchment area
during the period.
It may appear that the increased production of sediment on slopes (through humaninduced soil erosion) is being counter-balanced by trapping behind water conservation
impoundments. In other words the sediment delivery ratio (the proportion of sediment
reaching the outlet) has reduced. Modelling sediment delivery is one area that can be
usefully examined to expose the linkages between sediment production. By way of
example, a hypothetical model of sediment delivery was explored (Higgitt and Lu, [7]
that considered the time series of sediment delivered to a catchment outlet as the
catchment (represented through gridded cells) was progressively disturbed. The model
has two very simple operating rules. First, there is a pulse disturbance model where the
impact of the initial land cover transformation is marked by an acute increase in sediment
production which dissipates exponentially with time. The second component is a
sediment delivery effect where the proportion of material reaching the catchment outlet is
a function of distance. This rule reflects the increasing opportunity for sediments to
move into storage as the distance between source and destination increases. The model
demonstrates that when catchment moves in a wave away from the catchment outlet (as
for example where agricultural expansion moves progressively further up river valleys)
the initial increase in sediment yield quickly stabilises. During the model run the number
of cells that have been disturbed increases cumulatively, but the sediment yield generated
does not show an increasing tendency. This situation is similar to the reported
observations from erosion inventories in China that identify a substantial increase in the
areas that have been disturbed over time. An area that is designated as severely eroded
(following some previous period of deforestation) is not necessarily continuing to deliver
large sediment loads to the network.
Experimenting with simple sediment delivery models provides a partial explanation
for why the sediment yield signal from large basins is rarely as simple as the observed
land cover changes. Relationship between land cover change and sediment yield are
further complicated by the year to year variability in runoff and the difficulties of
accurately reconstructing land cover change. Understanding how changes in sediment
production and transfer feeds through to the catchment outlet is an important task for
predicting future sediment-related environmental problems, such as reservoir
sedimentation or nutrient fluxes.
SUMMARY
The spatial variability of land cover disturbance and of active soil erosion, combined with
time lag effects in the delivery of sediment through the catchment network provide a
partial explanation of why the sediment yield at a catchment outlet may not show a clear
trend in response to apparent land cover transformation and expected increases in
sediment mobility. Sediment transport through the outlet gauging station represents a
lumped response to the dynamics of mobility and transfer within the catchment.
Furthermore, the construction of large dams on main tributaries and the hundreds of
smaller water conservation structures within headwater catchments provide artificial
sediment sinks. From a management perspective, where a concern may be with the
quantity of sediment delivered into a large reservoir, or with nutrient flux to estuaries and
the oceans, estimation of the overall catchment sediment yield is the prime concern.
However, analysis of spatial and temporal variability of sediment fluxes within large
rivers indicates that the behaviour of the response variable (sediment yield at outlet)
masks considerable variation within the catchment, much of which is acting in
opposition. Large scale modelling of biogeochemical fluxes requires some of this
ambiguity to be resolved. A more sophisticated modelling of the time lags associated
with production and transfer of sediment associated with land cover disturbance, coupled
with analysis of residence time in storage would assist the prediction of sediment yield.
Distributed modelling within a GIS framework has much to offer but the role of field
geomorphology, particularly through rapid appraisal of erosion processes should not be
ignored.
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