EARTH SURFACE PROCESSES AND LANDFORMS
Earth Surf. Process. Landforms 39, 1855–1865 (2014)
Copyright © 2014 John Wiley & Sons, Ltd.
Published online 14 April 2014 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/esp.3573
Reappraisal of sediment dynamics in the Lower
Mekong River, Cambodia
XiXi Lu,1,2* Matti Kummu3 and Chantha Oeurng4
Department of Geography, National University of Singapore, Singapore
2
Global Change and Watershed Management Centre, Yunnan University of Finance and Economics, Kunming, China
3
Water & Development Research Group, Aalto University, Espoo, Finland
4
Department of Rural Engineering, Institute of Technology of Cambodia, Phnom Penh, Cambodia
1
Received 14 August 2013; Revised 18 March 2014; Accepted 18 March 2014
*Correspondence to: XiXi Lu, Department of Geography, National University of Singapore, Singapore. E-mail: [email protected]
ABSTRACT: The Mekong Basin in southeast Asia is facing rapid development, impacting its hydrology and sediment dynamics.
Although the understanding of the sediment transport rates in the Mekong is gradually growing, the sediment dynamics in the lower
Mekong floodplains (downstream from Kratie) are poorly understood. The aim of this study is to conduct an analysis to increase the
understanding of the sediment dynamics at the Chaktomuk confluence of the Mekong River, and the Tonle Sap River in the Lower
Mekong River in Cambodia. This study is based on the data from a detailed field survey over the three hydrological years (May
2008–April 2011) at the two sites (the Mekong mainstream and the Tonle Sap River) at the Chaktomuk confluence. We further
compared the sediment fluxes at Chaktomuk to an upstream station (i.e. Mukdahan) with longer time series. Inflow sediment load
towards the lake was lower than that of the outflow, with a ratio on average of 84%. Although annually only a small amount of
sediment load from the Tonle Sap contributes to the delta (less than 15%), its share is substantial during the February–April period.
The annual sediment load transport from the confluence to the delta in 2009 and 2010 accounted for 54 and 50 Mt, respectively.
This was on average only 55% of the sediment fluxes measured at Mukdahan, a more upstream station. Furthermore when compared
to sediment loads further downstream at the Cambodia–Vietnam border, we found that the suspended sediment flux continued to
decline towards the South China Sea. Our findings thus indicate that the sediment load to the South China Sea is much lower than
the previous estimate 150–160 Mt/yr. Copyright © 2014 John Wiley & Sons, Ltd.
KEYWORDS: sediment dynamics; suspended sediment; the Mekong River; the Tonle Sap River; Cambodia
Introduction
Global river systems have been increasingly altered by dam
construction and water diversions for water and energy needs
(Nilsson et al., 2005). Humans are simultaneously increasing
river sediment transport through soil erosion and decreasing it
through sediment retention in reservoirs (Syvitski et al., 2005).
The sediment load of a river provides an important measure
of its hydrology, morphodynamics, erosion and sediment delivery processes operating within its drainage basin (Walling,
2009). The magnitude of the suspended sediment load
transported by a river has important implications both for the
natural functioning of the river system, for example, through
its influence on channel morphology, water quality and aquatic
ecosystems, as well as habitats supported by the river, and for
human exploitation of the river system (Walling, 2009). The
suspended sediment load of a river is sensitive to both human
activities and climate change impacts within its drainage basin,
influencing erosion and sediment transfer into the river system.
Reservoirs have represented the most important influence to
the land–ocean sediment fluxes in the world (Walling and
Fang, 2003; Syvitski et al., 2005). They disrupt the continuity
of sediment transport and thus reduce the supply of sediment
to downstream reaches. Water released from a dam can be said
to be ‘sediment hungry’ and, under this circumstance a
sediment deficit may result in river degradation through bed
and bank erosion (Surian and Rinaldi, 2003; Surian and Cisotto,
2007). Quantification of sediment transport is a central part of
the general understanding of river morphology dynamics. The
understanding also helps to characterize and model associated
fluvial features and processes such as fish and invertebrate habitat, stability of infrastructures, water quality, reservoir sedimentation and coastline dynamics (Vericat and Batalla, 2010).
Quantifying sediment fluxes in catchments with existing and/or
planned reservoirs is fundamental baseline information to further
understand the possible impacts of sediment trapping on river
morphology and ecosystem functions (Tena et al., 2011).
The Mekong River Basin in southeast Asia is experiencing
dramatic land surface disturbances such as forest clearing, arable land expansion, reservoir construction and water diversion,
as a result of rapid economic and population growth (e.g. Lu
and Siew, 2006; Grumbine and Xu, 2011; Grumbine et al.,
2012; Lu et al., 2014), impacting on its hydrology (Johnston
and Kummu, 2012) and sediment dynamics (Kummu et al.,
2010). The annual flood pulse, driven by monsoon climate, is
essential for high productivity of aquatic ecosystems (Lamberts,
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X. LU ET AL.
2006) while sediment transport is critically important for
aquatic ecology, fisheries, agriculture, water supply and river
navigation (Wang J.J. et al., 2011). The existing and planned
reservoirs in the basin are estimated to trap a large part of the
river’s sediment load (Kummu et al., 2010) and thus cause
severe transboundary environmental impacts, which could
disrupt or result in the loss of important ecosystems and thus,
livelihoods of people living in downstream parts of the basin
(Lu and Siew, 2006).
Although the understanding of the sediment transport rates in
the Mekong is gradually growing (e.g. Lu and Siew, 2006;
Kummu and Varis, 2007; Walling, 2008; Kummu et al., 2008;
Kummu et al., 2010; Wang J.J. et al., 2011; Liu et al., 2013),
the sediment dynamics in the lower Mekong floodplains
(downstream from Kratie) are poorly understood. The lowest
Mekong mainstream station with reliable and long-term data
for suspended sediment concentration (SSC) is Mukdahan in
Thailand (Walling, 2008; Wang J.J. et al., 2011; Liu et al.,
2013). The lower Mekong floodplains contain the low land
from Kratie downstream in Cambodia (e.g. Västilä et al.,
2010) and potentially a large part of the Mekong’s sediment
flux is deposited there, as is happening in the Tonle Sap floodplains (Kummu et al., 2008) and does not reach the South
China Sea. Thus, the sediment flux dynamics of the basin to
the Mekong delta in Vietnam and finally to the South China
Sea, is not yet well understood.
Further, the Tonle Sap Lake and its floodplains connecting
with the Mekong mainstream through the Tonle Sap River are
also dependent on the Mekong sediment regimes since sediment transport from the Mekong is the main source of sediment
supply to the Tonle Sap Lake (Kummu et al., 2008). Sediment is
important to sustain the geomorphology of the floodplains and
particularly the Tonle Sap Lake and River, and to provide essential nutrients for the lakes’ productive ecosystem (Kummu et al.,
2008). The Tonle Sap floodplain is one of the most important
ecosystems in the Mekong River system (Lamberts, 2006;
Kummu et al., 2008; Arias et al., 2012) and within the most productive inland ecosystems in the world. However, the sediment
dynamics and connection between the Tonle Sap River and the
Mekong mainstream at Phnom Penh where the rivers join in a
confluence called Chaktomuk are poorly understood (see Figure 1). As the sediment input from the Mekong is crucial for
the Tonle Sap’s ecosystem functions (Arias et al., 2014), it
would therefore be extremely important to better understand
the sediment linkage between the Mekong mainstream and
the Tonle Sap River. The lack of holistic study on sediment dynamics between the Mekong mainstream and the Tonle Sap
River is mainly due to the lack of reliable long-term sediment
data. The only existing study quantifying sediment fluxes in
the Tonle Sap River (Kummu et al., 2008) used the data with
a large uncertainty (c.f. Walling, 2009).
With this study we thus aim to increase the understanding of sediment dynamics in the lower Mekong floodplains with data from a
detailed field survey over the period of 2008 to 2010 at the two
sites at the Chaktomuk confluence, Phnom Penh. The specific objectives of this study are firstly to examine the sediment dynamics at
the confluence of the Mekong River, and the Tonle Sap River at
Phnom Penh, secondly to quantify the sediment exchanges
between the Mekong mainstream and the Tonle Sap Lake, and
thirdly to quantify the sediment flux to the Chaktomuk confluence
from upstream and towards the Mekong delta from the confluence
Study Area
The Mekong River originates from the Tibet Plateau at an elevation of approximately 5000 m above the mean sea level. The
Copyright © 2014 John Wiley & Sons, Ltd.
river basin covers an area of 795 000 km2 and spans a total
length of 4800 km, running through China (Yunnan province),
Myanmar, Thailand, Lao PDR, and Cambodia before it finally
enters the South China Sea in southern Vietnam (Figure 1A).
The Mekong River basin is often divided into two parts: the
Upper Mekong Basin (UMB; 24% of total drainage area) and
the Lower Mekong Basin (LMB; 76% of total drainage area)
(e.g. Lu and Siew, 2006). The former is characterized by alpine
and mountainous areas with a low population density, while
the latter is characterized by low, flat topography with a high
population density (Iida et al., 2011). The river runs thus from
cool temperate to tropical climates from its origin to the South
China Sea. The melted snow waters from the high mountains
of the UMB control the downstream dry season flows while
the monsoon rains in the LMB dominates the wet season flow
(Darby et al., 2013).
The hydrology of the Mekong River is characterized by a
mono-modal flood pulse. The water level usually begins rising
in May and peaks in September or October, with an average
peak flow of 45 000 m3/s (Lu and Siew, 2006). Around November,
the water level starts to recede and reaches the lowest levels in
March and April, at approximately 1500 m3/s (Kite, 2001). The
Mekong River is Asia’s third largest river in terms of length and
sediment load, delivering approximately (based on stations in
Southern Lao PDR and Eastern Thailand) 160 million tons (Mt) of
sediment and 475 km3 of fresh water per year into the South China
Sea (Milliman and Meade, 1983).
The Mekong River is connected to the Tonle Sap Lake via the
Tonle Sap River at Phnom Penh, Cambodia (Figures 1B and
1C). The Tonle Sap Lake is the largest body of freshwater in
southeast Asia, and forms a key part of the Mekong floodplains
hydrological system. Its mean surface area varies from an average of 2300 km2 in May to a maximum of over 15 000 km2
during the peak of the wet season (Kummu and Sarkkula,
2008; Kummu et al., 2014). The water level rise in the Mekong
mainstream forces the Tonle Sap River to reverse its ‘normal
flow’ (from the lake towards the Mekong) to the Mekong
towards the lake, where the water level is lower than that of the
Mekong River. This phenomenon occurs annually in May–June.
In September–October, when the water level in the Mekong starts
to recede, this stored water starts to drain back into the lower Mekong and Bassac Rivers (Masumoto, 2000; Fuji et al., 2003). The
lake receives most of its water from the Mekong mainstream and,
as the lake’s water level is directly controlled by the water level of
the Mekong (Kummu et al., 2014). This natural mechanism provides a peculiar and important balance to the Mekong River
downstream from the lake and ensures a freshwater flow into
the Mekong Delta in Vietnam during the dry season, protecting,
for example, the rich agricultural lands of the delta from saltwater
intrusion from the South China Sea (Pham et al., 2008).
Data and Methods
Water discharge and suspended sediment
concentration (SSC) measurement
We used two established water level stations as our study sites:
Chroy Changvar at the Mekong mainstream, and Phnom Penh
Port at the Tonle Sap River (see locations in Figure 1C and
cross-sections in Figure 1D). The daily water level data are
based on the Mekong River Commission (MRC) database.
Water discharges at both sites have been generated with rating
curves created by MRC. We conducted sampling of the river
water quality, including SSC, at these two sites for three years
from May 2008 to April 2011.
Earth Surf. Process. Landforms, Vol. 39, 1855–1865 (2014)
SEDIMENT DYNAMICS IN THE MEKONG RIVER
1857
Figure 1. Study site maps. (A) Mekong River Basin with the upstream measurement stations; (B) lower Mekong floodplains with the locations of the
observation sites of this study; (C) detail map of the Chaktomuk confluence showing the locations of the measured cross-sections; (D) cross-sections of
the observed locations. This figure is available in colour online at wileyonlinelibrary.com/journal/espl
Water sampling was undertaken at three different profiles
within the cross-section: at 1/4 of the width from the left bank,
in the middle, and 1/4 of the width from the right bank of the
river. We used a horizontal 1000 ml Van Dorn sampler to take
the three samples from the surface (0.5 m below water level),
0.2 × water depth and 0.8 × water depth at each of the three
sampling profiles (Supplementary Material Figure S1). The
three samples from the three layers were then mixed before filtering. According to earlier suspended sediment measurements
campaigns supported with various Acoustic Doppler Current
Profilers (ADCPs), conducted in 2005–2006 within WUP-FIN
project under MRC, there are no major differences in SSCs
within the profile (Supplementary Material Figure S2).
The measurements were taken with high regularity during
flood periods (four to six times per month), but lower frequency
during dry season. In total 273 water samples (91 × 3 points) were
taken for both sites during the study period. The samples for SSC
were filtered in pre-cleaned 0.45 μm pore size, 47-mm-diameter
Copyright © 2014 John Wiley & Sons, Ltd.
pre-weighed Nuclepore filters. The filters were dried at 60 °C for
24 hours and weighed for quantifying SSC. The SSC values used
in this study correspond to the mean of three samples collected
at three different points in the river cross-section. The variations
of the SSC values within a cross-section are shown in the
Supplementary Material Figure S3. To further validate our SSC
measurements, we compared our measurements with the MRC
measurements of ‘D-96 Flow proportional depth integrated
suspended sediment sampler’ (Koehnken, 2012; MRC, 2013),
which overlapped our campaign for year 2010 (Supplementary
Material Figure S4).
Sediment rating curve and sediment load estimation
The daily SSC measurements were used to develop sediment
rating curves, which depict the statistical relationship between
daily SSC and daily discharge. The widely applied sediment
Earth Surf. Process. Landforms, Vol. 39, 1855–1865 (2014)
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X. LU ET AL.
rating curve, based on the power regression between water discharge and SSC (e.g. Walling, 1977, and references cited
therein), is generally expressed as:
C s ¼ aQ b
(1)
where Cs denotes the SSC (in mg/l), Q denotes the water discharge (in m3/s) and a and b are sediment rating coefficient
and exponent, respectively. We selected to use the rating curve
instead of an interpolation method due to the rather low sampling frequency in some parts of the sampling campaign. We
did, however, compare the rating curve results with interpolation method (Supplementary Material Figure S5), which used
the measured SSC values to estimate SSC for the days which
we did not have measurements. The values were interpolated
by linear equation from the two closest sampling dates.
For the Mekong site, we divided the hydrological year into
the two phases: the rising stage of the flood (May–September)
and the falling stage of the flood (October–April) (Table I;
Figure 2A). For the Tonle Sap River site, the sediment rating
curve was separated into two separate periods following Tonle
Sap inflow (into the lake) and outflow (out from the lake) for
each hydrological year (Figure 2B). These follow rather well
the rising and falling limbs of the Mekong mainstream with
small interannual differences.
Daily SSC values were estimated using the sediment rating
curve (Figure 2) and discharge at a given station. The sediment
load (SL; metric tons/day) was computed from the estimated
sediment concentration (Cs) and the measured water discharge
(Q): SL= Q × Cs.
Results
Water discharge and suspended sediment dynamics
at Chroy Changvar of the Mekong River
The hydrograph of the daily water discharge showed obvious
seasonal variation governed by a distinct dry season and a
rainy season each year (Figure 3). During the low flow period
in April and May, the discharge stayed at approximately
1500 m3/s. Responding to the rainy season, the discharge
started to increase in June and reached a peak in August or
September. After the annual maximum, the discharge steadily
decreased to return to the low level. During the observation
period (May 2008 to April 2011), maximum discharge reached
35 599 m3/s while minimum discharge was 1331 m3/s. The
water yield for hydrological year 2010 (290 km3) was much
lower than that in 2008 (416 km3) or 2009 (380 km3). Mean water
yield for the whole study period amounted to 362 km3/yr. The
maximum SSC during the three year sampling period reached
571 mg/l, observed on 20 August 2008. Discharge-weighted
mean SSC for the whole study period was 225 mg/l. The Q and
SSC values showed significant statistical relationships during
the study period (R2 = 0.81; p < 0.05). Seasonal relationships
were also analysed by dividing the dataset into rising or falling
stages of the flood periods. It can be seen that the SSC dynamics
Table I. Timing of the rising and falling stages, used in this study, of
the Mekong River.
Rising stage
May 2008–September 2008
May 2009–September 2009
May 2010–September 2010
Copyright © 2014 John Wiley & Sons, Ltd.
Falling stage
October 2008–April 2009
October 2009–April 2010
October 2010–April 2011
have interannual variation and thus, it is justified to use the seasonal rating curves, separate ones for each year, to calculate the
sediment loads. The SSC was generally higher during the rising
stage of the flood compared to the falling one (Figure 2A). In general the Q–SSC relationship was better during the falling stage of
the flood period (October–April) (Figure 2A).
The analysis of temporal sediment dynamics also focuses on
hysteresis effects. The behaviour of suspended sediment and
changes in SSC during flood events are not only a function of
energy conditions, i.e. sediment is stored at low flow and
transported under high-flow conditions, but are also related to
variations in sediment supply and sediment depletion. These
changes in sediment availability result in so-called hysteresis
effects (Asselman, 1999). These patterns reflect the combination of sediment supply from dominant sources with the capacity of flows to transport the supplied sediment to the catchment
outlets. Sediment delivery processes can be interpreted using
suspended sediment-discharge hysteresis patterns (Oeurng
et al., 2010).
During the annual flood of the three-year observation
periods, the relationship between SSC and water discharge
was dominated by a clockwise hysteretic loop in the Mekong
River at Chroy Changvar, with the concentration for a given discharge being higher on the rising limb of the hydrograph than
that for the similar/same discharge in the falling limb (Figure 4).
Some examples can be seen on the rising flood observed on 31
July 2008 (Q = 25 347 m3/s; SSC = 450 mg/l) but receding flood
(Q = 25 447 m3/s; SSC = 269 mg/l) was observed on 9 September 2008. In 2009, the rising flood of 28 543 m3/s on 20 July
2009, attained an SSC of 365 mg/l while the receding flood
(Q = 28 339 m3/s; SSC = 121 mg/l) was recorded on 30 September 2009. It was also the same case for hydrological year 2010.
The intra-annual variability of water yield and sediment load
in the Mekong River was large, ranging from a minimum load
of 0.1 Mt to a maximum of 30.8 Mt (Table II). The monthly maximum sediment load accounted for 30.8 Mt in August 2008,
15.4 Mt in August 2009 and 16.1 Mt in September 2010. A
large part of the sediment load was transported in August and
September, accounting together for around 50–60% of total
annual load (Table II). During the dry season, March and April
yielded the lowest loads of sediment. Monthly sediment load
was found to be well-correlated with monthly water yield,
resulting in a significant regression (R2 = 0.90). The annual
sediment load transport in hydrological years 2008, 2009,
and 2010 were 91 Mt, 54 Mt, and 50 Mt, respectively.
Water discharge and suspended sediment dynamics
in the Tonle Sap River
The hydrograph of the daily water discharge of the Tonle Sap
River showed that the peak inflow into the lake generally
occurred in the same period as the peak discharge of the Mekong
River (August–September); whereas, peak outflow from the lake
mainly took place from November to December, a few months
after the peak inflow (Figure 5). During the observation period,
maximum inflow discharge into the lake accounted for 7032 m3/s
while the minimum was 104 m3/s. For water flowing out from
the lake, the maximum outflow amounted to 8176 m3/s while
the minimum was 380 m3/s. The mean water inflow for the study
period accounted for approximately 40% of water outflow.
The temporal dynamics of the sediment transport in the
Tonle Sap River is governed by two flow directions (inflow from
May to September and outflow from October to April), showing
the temporal variations of SSC during the observation period
(Figure 4). It can be seen that SSC during the inflow period is
Earth Surf. Process. Landforms, Vol. 39, 1855–1865 (2014)
SEDIMENT DYNAMICS IN THE MEKONG RIVER
1859
A. Rating curves at Chrui Changvar (Mekong mainstream)
Falling stage 2008
Rising stage 2008
SSC (mg/l)
600
y = 0.0028x1.1399
R2 = 0.850
400
y = 0.0114x0.9283
R2 = 0.921
200
0
Rising stage 2009
Falling stage 2009
SSC (mg/l)
600
y = 0.0088x
R2 = 0.948
y = 0.0068x
R2 = 0.885
400
200
0
Rising stage 2010
Falling stage 2010
SSC (mg/l)
600
y = 0.0112x
y = 0.0229x0.8818
R2 = 0.756
400
R2 = 0.935
200
0
0
10,000
20,000
30,000
40,000
0
10,000
3
20,000
30,000
40,000
3
Water discharge (m /s)
Water discharge (m /s)
B. Rating curves at Phnom Penh Port (Tonle Sap River)
Outflow from Tonle Sap 2008
Inflow into Tonle Sap 2008
SSC (mg/l)
600
y = 0.0672x1.0001
R2 = 0.826
400
y = 7.1049x0.3490
R2 = 0.240
200
0
Inflow into Tonle Sap 2009
Outflow from Tonle Sap 2009
SSC (mg/l)
600
y = 0.1488x0.7360
R2 = 0.846
y = 0.4967x0.6943
R2 = 0.799
400
200
0
Inflow into Tonle Sap 2010
Outflow from Tonle Sap 2010
600
y = 167.82x-0.0450
R2 = 0.844
SSC (mg/l)
0.0362x1.0141
y=
R2 = 0.927
400
200
0
0
2000
4000
6000
8000
0
2000
3
4000
6000
8000
3
Water discharge (m /s)
Water discharge (m /s)
Figure 2. Sediment rating curves based on field observations. (A) Sediment rating curves for rising and falling stages of the Mekong River at Chroy
Changvar; (B) sediment rating curves for the inflow and outflow periods for the Tonle Sap River at Phnom Penh Port. This figure is available in colour
online at wileyonlinelibrary.com/journal/espl
Hydrological year 2010
800
40,000
SSC
600
30,000
400
20,000
200
May-11
Nov-10
May-10
Nov-09
May-09
0
Nov-08
10,000
SSC (mg/l)
Water discharge
May-08
Discharge (m3/s)
Hydrological year 2009
Hydrological year 2008
50,000
0
Figure 3. Temporal variation in the water discharge (left y-axis) and suspended sediment concentration (SSC; right y-axis) at Chroy Changvar, Mekong
River (see location in Figure 1) for hydrological years 2008–2010. This figure is available in colour online at wileyonlinelibrary.com/journal/espl
Copyright © 2014 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms, Vol. 39, 1855–1865 (2014)
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X. LU ET AL.
SSC (mg/l)
May 2008- April 2009
May 2009-April 2010
600
600
400
400
400
200
200
200
0
0
0
0
10,000
20,000
30,000
40,000
May 2010-April 2011
600
0
10,000
Water discharge (m3/s)
20,000
0
30,000
10,000
Water discharge (m3/s)
20,000
30,000
Water discharge (m3/s)
Figure 4. The relationship between water discharge and suspended sediment concentration (SSC) dominated by clockwise hysteresis for each
observed hydrological year (2008, 2009, and 2010).
Table II. Temporal variability of monthly water yield and monthly sediment load for the study period of May 2008 to April 2011 at Chroy Changvar
of the Mekong River.
2008
Month
May
June
July
August
September
October
November
December
January
February
March
April
Total
3
2009
2010
3
3
Water yield (km )
SS load (Mt)
Water yield (km )
SS load (Mt)
Water yield (km )
SS load (Mt)
14.7
36.3
54.3
82.9
73.3
58.5
40.3
22.7
12.0
7.7
6.4
7.0
416
0.91
6.12
12.85
30.78
24.59
8.97
4.33
1.38
0.37
0.17
0.11
0.13
91
12.8
28.3
57.2
77.4
70.0
68.0
27.6
13.8
8.4
5.8
5.3
5.7
380
0.60
2.40
9.45
15.35
13.06
10.11
1.81
0.45
0.17
0.09
0.07
0.08
54
7.0
10.5
20.0
56.3
68.7
58.7
30.2
15.9
8.1
5.0
4.2
5.0
290
0.16
0.50
1.59
10.72
16.15
14.61
4.02
1.11
0.28
0.12
0.08
0.92
50
Hydrological year 2008
Hydrological year 2009
Hydrological year 2010
10000
OUTFLOW
600
400
6000
4000
200
2000
0
0
-2000
-200
-4000
-6000
-400
Water discharge
Nov-10
May-10
Nov-09
INFLOW
May-09
Nov-08
May-08
-10000
SSC
May-11
-8000
SSC (mg/l)
Water Discharge (m3/s)
8000
-600
Figure 5. Temporal variations in the water discharge (inflow and outflow) and suspended sediment concentration (SSC) at Phnom Penh Port, Tonle
Sap River (May 2008– April 2011). Negative values are inflow into the lake while positive values correspond to the outflow from the lake. This figure is
available in colour online at wileyonlinelibrary.com/journal/espl
higher than that during the outflow period since inflow SSC originated from the Mekong water, containing high SSC. Peak inflow
discharges generally have much higher SSC than those with similar peak outflow discharges. A relationship between SSC and
water discharge for inflow period from the Mekong is well correlated (R2 = 0.74) while being much lower during the outflow from
the lake (R2 = 0.31). The seasonal relationship for the Tonle Sap
River was also examined during inflow and outflow periods. It
can be observed that the Q–SSC relationship was good with a
steep slope during inflow from the Mekong River into the lake,
while a flat slope during outflow from the lake (Figure 2B).
During our study period, the inflow into the lake took place
normally between June and August or September. The maximum sediment load into the lake was observed normally in
Copyright © 2014 John Wiley & Sons, Ltd.
August while maximum sediment load from the lake took place
in November (Figure 5). The maximum monthly inflow
sediment load yielded 5.7 Mt in August 2008, 2.4 Mt in August
2009, and 2.2 Mt in August 2010 (Figure 6). These amounts
represent 43–65% of the total annual inflow sediment load.
During the outflow period, the maximum monthly sediment
load accounted for 4.5 Mt in December 2008, 2.3 Mt in
November 2009, and 2.0 Mt in November 2010 (Figure 6).
These represent 30–40% of the total annual sediment load
transported out from the lake. Inflow of both, water yield and
sediment load, is lower than that of the outflow, representing
a ratio ranging from 0.39 to 0.47 for water yield (on average
44% of water outflow) and 0.59 to 0.99 for sediment load (on
average 84% of sediment outflow) (Table III).
Earth Surf. Process. Landforms, Vol. 39, 1855–1865 (2014)
SEDIMENT DYNAMICS IN THE MEKONG RIVER
Hydrological year 2008
Hydrological year 2010
10
0
Nov-10
Nov-09
May-10
INFLOW
May-09
Nov-08
May-08
Inflow Sediment load
May-10
Nov-09
Outflow Sediment load
-4
May-09
Flux away from Mekong delta
-10
-2
Nov-08
0
20
May-08
Sediment load (Mt)
2
Hydrological year 2010
Flux towards Mekong delta
Flux towards Mekong delta:
From Tonle Sap
From Mekong
May-11
Sediment load (Mt)
4
-6
Hydrological year 2009
30
OUTFLOW
May-11
Hydrological year 2009
6
Nov-10
Hydrological year 2008
1861
Flux away from Mekong delta:
To Tonle Sap
Figure 7. Monthly sediment flux dynamics of the Chaktomuk Confluence, separated into three components: the Mekong mainstream at
Chroy Changvar, the Tonle Sap inflow and the Tonle Sap outflow.
Positive values correspond to the flux towards the Mekong delta, while
the negative values correspond to the flux away from the delta (i.e.
towards Tonle Sap Lake). This figure is available in colour online at
wileyonlinelibrary.com/journal/espl
Figure 6. Monthly sediment inflow and outflow observed at Phnom
Penh Port of the Tonle Sap River for the period of May 2008 to April
2011. This figure is available in colour online at wileyonlinelibrary.
com/journal/espl
Sediment linkage between the Mekong mainstream
and the Tonle Sap River and sediment flux towards
the Mekong delta
Discussion
In this article we quantified, for the first time, the sediment flux in
the Mekong that enters the Mekong Delta from the Chaktomuk
junction, Phnom Penh, Cambodia. So far, most downstream
stations of reliable estimates for the sediment flux have been
Mukdahan and Khong Chiam in Thailand (Walling, 2008; Wang
J.J. et al., 2011; Liu et al., 2013). Our estimates suggest that the
sediment flux into the South China Sea is much lower than
estimated based on its greater number of upstream stations
(~150–160 Mt/yr), as the average of a three-year measurement
period at Phnom Penh, entering towards the delta, was 67 Mt/yr.
This is, however, based on only a few years of measurements but
when compared to the measurements of the same year in
Mukdahan (Figure 8), we can get the first estimates for the sediment
dynamics between Mukdahan and the upstream boundary of the
Mekong Delta. Moreover, part of that sediment might still end up
in the floodplains from Phnom Penh towards the South China
Sea. This is estimated to be approximately 6–10% of the sediment
load transported in the river on the Cambodian–Vietnam border
(Manh et al., submitted for publication).
The sediment loads of the Mekong River achieve the highest in
the section from Nong Khai-Mukdahan-Khong Chiam (Figure 8;
see also Liu et al., 2013). After this section, the river enters the
depositional environment. Although the 3S rivers downstream of
Khong Chiam contribute around 15% of the total sediment to the
Mekong River (Koehnken, 2012), this section of the river basin is
predominantly occupied by flood plains, resulting in a dramatic
decline in the sediment loads from around 150 Mt/yr in Mukdahan
to well below 100 Mt/yr in Phnom Penh. Secondly, the linkage
between the Mekong mainstream and the Tonle Sap River was
studied in this paper. We found that their inflow and outflow are
The linkage between the Mekong mainstream and the Tonle
Sap River is connected at the Chaktomuk confluence at Phnom
Penh, Cambodia. Inflow from the Mekong River into the Tonle
Sap Lake has naturally rather similar sediment concentration to
the ones in the Mekong River. The monthly inflow sediment
load to the Tonle Sap Lake during our three-year study had
statistically significant relationship with the monthly Mekong
sediment load (R2 = 0.94; p < 0.05).
During the flood season, when the Tonle Sap River flows into
the Tonle Sap Lake, the sediment load from the Chaktomuk
confluence towards the South China Sea is reduced by
the amount of sediment flux that flows into the lake. During the dry season, when the Tonle Sap River reverses back
to the Mekong, the sediment flux out from the lake adds to
the Mekong sediment load towards the South China Sea.
Monthly sediment load components at the Chaktomuk
confluence are presented in Figure 7. Although annually
the sediment load from the Tonle Sap contributes only a
small amount of the sediment to the delta (on average less
than 15% of the total flux into the delta comes via the
Tonle Sap River), its monthly share is substantial if compared with the monthly sediment load from the Mekong
River during the December–April period, reaching 88% of
total sediment load to the delta in 2008, 77% in 2009
and 58% in 2010, respectively. Sediment flux combining
Mekong sediment load and the Tonle Sap sediment load
from the Chaktomuk confluence towards the Mekong delta
accounted for 97 Mt in 2008, 54 Mt in 2009, and 50 Mt in
2010, respectively.
Table III. Annual water yield and sediment load of inflow into Tonle Sap Lake and outflow from the lake, and inflow/outflow ratio at Phnom Penh
Port, Tonle Sap River for hydrological years 2008–2010.
Inflow
Hydrological
year
2008
2009
2010
Mean
3
Outflow
3
Ratio (inflow/outflow)
WY (km )
SL (Mt)
WY (km )
SL (Mt)
31.0
32.9
26.0
30.0
8.8
5.7
4.6
6.3
80.0
71.0
55.0
68.7
14.9
6.0
4.6
8.5
WY
SL
0.39
0.46
0.47
0.44
0.59
0.95
0.99
0.84
Note: WY, water yield; SL, sediment load.
Copyright © 2014 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms, Vol. 39, 1855–1865 (2014)
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X. LU ET AL.
Luang Prabang
Vietnam Delta
Nong Khai
Chiang Saen
Mukdahan
Phnom Penh
250
UMB
LMB
Sediment load [Mt/yr]
200
150
2011
100
2010
2008
2009
2011
2009
50
2010
0
200,000
400,000
Range for years 1993-2003,
based on literature
(Walling 2008, Wang et al 2011
and Liu et al 2013):
max
mean
min
600,000
This study
MRC data
Manh et al
0
800,000
Basin area [km2]
2008:
2009:
2010:
2011:
Figure 8. Comparison of our results to the upstream average values over 1993–2003 (Walling, 2008; Wang J.J. et al., 2011a; Liu et al., 2013), recent
sediment fluxes at Mukdahan for years 2009–2011 (MRC, 2013) and downstream sediment fluxes at Cambodia–Vietnam border (Manh et al., submitted for publication). MRC (2013) data is based on D-96 Flow proportional depth integrated suspended sediment sampling while Manh et al. (submitted for publication) used point-integrated SSC observations (one profile, five depths) by Southern Regional Hydro-Meteorological Centre of Vietnam.
This figure is available in colour online at wileyonlinelibrary.com/journal/espl
somewhat in balance, their outflow being slightly larger than their
inflow. Both of these issues are further discussed later.
Verification of the measurements and calculations
We acknowledge that the method we used for measurements
(i.e. point-integrated sampling method) might not catch the
SSC variations within the profile, when compared to the
depth-integrated method. For example, when the SSC from
different depths (Supplementary Material Figure S2) or backscatter of the ADCP from the high flood are examined
(Supplementary Material Figures S6 and S7), one can see that
the sediment concentration (high backscatter correlate with
high SSC) varies within a profile.
Therefore, we verified our measurements based pointintegrated sampling method against the MRC measurement campaign results, which were based on ‘D-96 Flow proportional
depth integrated suspended sediment sampler’. The comparison
of our SSC results for year 2010 revealed that our measurements
are rather well in line with theirs (Supplementary Material Figure
S4). During the rising flood and peak flood the values were very
similar, while during the receding flood our values for SSC were
slightly higher than theirs. Therefore, although the method we
used had some limitations, it seems to result in reliable SSCs.
For the future campaigns, however, we recommend the coupling
of the point-integrated measurements with the ADCP measurements, so that the heterogeneity of the SSC over the whole
cross-section can be obtained.
Copyright © 2014 John Wiley & Sons, Ltd.
We further verified our sediment load calculations based on
the rating curve approach by comparing these with the fluxes
estimated using the interpolation method. This verification revealed that there is not a large difference between the two
methods, as the use of rating curve method resulted in a mean
annual sediment load in the Mekong mainstream at Chroy
Changvar of 65 Mt/yr, while the interpolation method resulted
in a load of 66 Mt/yr (Supplementary Material Table S1). There
was, however, a difference in monthly values as the rating
curve method resulted slightly lower values for the rising flood
and higher values for the receding flood when compared with
the interpolation method (Supplementary Material Figure S5).
Sediment linkages between upstream and
downstream of the Chroy Changvar
The seasonal relationship between water flow and SSC in the
Mekong River offers interesting insights into temporal patterns
of sediment load. Sediment transport in most of the large river
systems increases with precipitation and water discharge
(Trenhaile, 1997). For the Mekong River, most of the sediment
load was transported during the peak flood in August and
September. The temporal variability of sediment transport
in the Mekong River can be well explained by the variability of the water flow in the river with significant linear
relations (R2 = 0.90; p < 0.05). The relation between discharge and sediment load in hydraulic-dependent fluvial
systems tends to be direct (Tena et al., 2011).
Earth Surf. Process. Landforms, Vol. 39, 1855–1865 (2014)
SEDIMENT DYNAMICS IN THE MEKONG RIVER
Although, in general terms, the relation between discharge
and sediment load in the Mekong River at Phnom Penh shows
a hydraulic-dependent pattern, as in the three hydrological
years analysed. In the hydrological year 2008, the sediment
load was considerably higher than in 2009 and 2010, reflecting
its high water yield. The mean annual sediment load of the
Mekong River entering the Phnom Penh from the upstream
during the three-year observation period was 65 Mt/yr, and
when the Tonle Sap River contribution (2 Mt/yr) was taken into
account, the average annual flux towards the Mekong Delta was
67 Mt/yr. This value is considerably lower than sediment loads of
the upstream stations reported in the literature (Walling, 2008;
Wang J.J. et al., 2011; Liu et al., 2013). For example, Mukdahan,
the most downstream station with a reliable long-term dataset
above Phnom Penh, has a sediment load of 140 Mt/year during
the period 1993–2003 (Liu et al., 2013). Naturally, the observed
period is different and thus no direct comparison can be made.
When we compared, however, the sediment loads in
Mukdahan (MRC, 2013) to the ones in Phnom Penh, during the
two years overlapping our observations and MRC observations
(2009 and 2010), we found that the sediment flux downstream
(i.e. Phnom Penh) was only around 55% of that upstream (see
Figure 8). MRC (2013) measurements in Mukdahan and Phnom
Penh were based on ‘D-96 Flow proportional depth integrated
suspended sediment sampler’ accompanied with ADCP measurements. Sampling frequency was around 35–40 samplings
per year in Mukdahan (years 2009–2011) and somewhat less in
Phnom Penh (year 2011 only). The sediment fluxes were
estimated by using daily discharge and sediment rating curve.
We further compared the Phnom Penh value in these two
years (52.2 Mt/yr) to the sediment load entering the delta
(average over 2009 and 2010 was 41.3 Mt/yr; representing
the sum of the sediment fluxes at Tan Chau and Chau Doc).
The sediment flux estimates were based on daily discharge
and daily point-integrated SSC observations (one profile, five
depths) by Southern Regional Hydro-Meteorological Center of
Vietnam, as summarized in Manh et al. (submitted for publication). This comparison indicates that the sediment load further
decreased towards the delta (Figure 8). A similar trend was observed for the year 2011, based on observed sediment loads in
Mukdahan (MRC, 2013) and the ones entering the Mekong
Delta (Manh et al., submitted for publication) (Figure 8).
However, it should be noted that bedload was not measured in
our study and it might account for a significant quantity of the sediment that was transported in suspension at Mukdahan upstream.
Koehnken (2012) indeed found that in MRC grain size analyses
the sand particles represented the largest share of the suspended
sediment flux in Pakse, while in Kratie, Cambodia (see locations
in Figure 1B) no sand particles were found in suspended flux.
Koehnken (2012) reported that the bed load for year 2011 in Kratie
was only 1.6 Mt/yr, approximately 1.4% of the suspended load of
the same year. This indicates that either sand is deposited on river
floodplains (between Mukdahan and Kratie) or it is not tracked
with either bedload or suspended sediment samplers.
The reduction of sediment load between Mukdahan and
Phnom Penh could be attributed to deposition and storage of
sediment within the channel and the floodplain system between
these two stations. According to Gupta and Liew (2007), most of
the sediment in the Mekong upstream of Cambodia appears to
be stored inside the channel, either on the bed or as insets against
rock-cut banks. The sediment load in Mukdahan of the compared
years (2009–2011) was below the average sediment loads of 1993
to 2003 (Figure 8), while the discharges were close to the longterm average of 7600 m3/s (MRC, 2005). Therefore, we were not
able to assess in this paper how the sediment dynamics would
look at the Cambodian part of the Mekong during high flood or
very dry conditions.
Copyright © 2014 John Wiley & Sons, Ltd.
1863
There are some indications that sediment load at the Gaiju
station, in the Chinese part of the Mekong, has decreased since
the operation of the Manwan Dam in 1993 (Wang H. et al.,
2011; Liu et al., 2013), based on comparison of years
1965–1992 and 1993–2003. Liu et al. (2013) did not, however,
observe any substantial decrease in sediment load downstream
of that station (Wang J.J. et al., 2011; Liu et al., 2013). It is
suggested that this was likely due to deforestation in Laos
and Thailand that has increased the sediment load from that
part of the basin (Wang J.J. et al., 2011). Since 2003,
though, many more dams have been put into operation
and the reservoir capacity has increased considerably (e.g.
Kummu et al., 2010; Johnston and Kummu, 2012). However, the impact of the recently built reservoirs on the sediment
load has not been assessed. Kummu et al. (2010) estimated that a
large portion (around 60–80%) of the basin’s sediment load
could be trapped, should the planned reservoirs be built,
however, they did not take into account the geomorphological
responses, e.g. increased bank erosion, due to decreased
suspended sediment load.
Sediment exchange between the Tonle Sap River
and the Mekong mainstream
The relationship between sediment and water discharge in
the Tonle Sap River showed a good correlation during the
inflow period (water flow from the Mekong into the Tonle
Sap Lake), but rather weak relation while reversing flow
from the lake into the Mekong. Sediment load from the
Mekong mainstream was high during the flooding period
and entirely controlled by the Mekong flow regime. Sediment concentrations during the outflow from the lake were
much lower compared to the water discharges from the Mekong
River. The sediment flow from the Mekong was generally less variable because its watershed is much larger than that of the Tonle
Sap Lake. The outflow from the lake had much higher variability
than the inflow as it was influenced by local behaviour from reversing flow from the lake.
Even though outflow sediment concentration in the Tonle
Sap River was generally smaller than the inflow concentration,
the annual sediment outflow was higher than sediment inflow
for all the three years studied. This can be explained by higher
water yield within the outflow period (inflow was only 44% of
water outflow) as the outflow includes the water originating
from the Tonle Sap catchment with an area of 90 000 km2.
Although a large part of the sediment deposited into the lake’s
floodplain (Kummu et al., 2008), the sediment originating from
the Tonle Sap catchment increases the total sediment load
entering the lake. Kummu et al. (2008) estimated that over the
period 1997–2003 the lake system received sediment from
the Mekong on average 5.1 Mt/yr and from its own tributaries
2.0 Mt/yr. Therefore, around one quarter of the sediment entering the lake originates from the tributaries.
The mean sediment inflow into the lake from the Mekong
mainstream calculated in this study (6.3 Mt) is close to the
mean value of 5.1 Mt reported by Kummu et al. (2008). Their
analysis is based on monthly surface water measurements
obtained from the MRC water quality database during the years
1997–2003. In contrast, the sediment outflow (7 Mt/yr)
observed in our study is not consistent with the previous result.
We found that the sediment outflow was higher than the inflow;
whereas Kummu et al. (2008) previously reported a low mean
sediment outflow of only 1.4 Mt/yr exported from the lake during their study period (1997–2003). Walling (2009) reported
that the Water Quality data of the Mekong Programme greatly
Earth Surf. Process. Landforms, Vol. 39, 1855–1865 (2014)
1864
X. LU ET AL.
underestimate the true concentrations. Therefore, the sediment
load estimated by Kummu et al. (2008) might also be an
underestimation.
There are, however, various reasons that might have an impact on the inconsistency between our study and Kummu
et al. (2008). First, the measurements stations were different.
In our study the sediment load was measured at the Phnom
Penh port, whereas with Kummu et al. (2008) the Prek Kdam
station, which is located some 60 km towards the lake from
the port, was used. Between these stations a floodplain
connected to the Mekong mainstream (upstream from Phnom
Penh) enters the Tonle Sap River and thus might transport
sediment to the Tonle Sap River during the flood events in the
Mekong River. Second, while this study is based on pointintegrated SSC measurements, Kummu et al. (2008) used a
single-point surface water sample measuring total suspended
solids (TSSs). Third, while we sampled during the flood period
several times per month, the MRC water quality database includes only monthly measurements throughout the year. However, within the MRC depth-integrated measurement campaign
for year 2011, the inflow (6.4 Mt/yr) and outflow (1.5 Mt/yr) at
Prek Kdam (Koehnken, 2012) were in line with Kummu et al.
(2008). Therefore, a more detailed study should be undertaken
to fully understand the sediment dynamics between the lake
and the Mekong mainstream.
Sediment load entering the Tonle Sap River through the
connection with the Mekong mainstream is highly dependent
on the Mekong sediment regimes. Hence, future dam impacts
(i.e. reservoir trapping) along the Mekong mainstream can
also impact the lake’s sediment dynamics. Arias et al.
(2014) simulated that a reduction in the sediment flux into
the Tonle Sap Lake, together with changes in flood pulse
due to hydropower operation, could decrease notably the
lake’s productivity. This might further have dramatic effects
on the lake’s very productive fisheries. Moreover, changes
in natural sediment dynamics would threaten the longerterm stability of the Mekong delta (Campbell, 2007; Saito
et al., 2007).
Conclusion
In this study we examined the sediment dynamics in the
Mekong mainstream at Phnom Penh and the sediment
exchange between the Tonle Sap River and the Mekong
mainstream during a three-year observation period
(2008–2010). We conducted a detailed field campaign
during May 2008–April 2011 at the two sites, one in the
Mekong mainstream and one in the Tonle Sap River. Seasonal sediment rating curves were applied to quantify the
sediment load at both sites.
We found that the monthly variability of sediment load in the
Mekong River encompasses a strong variability following the
discharge pattern rather closely, ranging from a minimum
monthly load of 0.1 Mt to a maximum of 30.8 Mt during the
study period. The annual sediment load in the Mekong varied
between 50 Mt/yr and 91 Mt/yr. When we compared the sediment loads in the Phnom Penh to the one upstream station
Mukdahan, we found a drastic decrease of 45%. The sediment
flux further decreased approximately 20% towards the Mekong
Delta at the Cambodia–Vietnam border. Our findings thus indicate that the sediment load to the South China Sea is much
lower than previously estimated (150–160 Mt/yr). To get more
reliable estimates, a longer observed period would be needed
covering very dry and wet years.
Sediment exchange between the Mekong mainstream and
the Tonle Sap River is connected at the Chaktomuk confluence
Copyright © 2014 John Wiley & Sons, Ltd.
in the Phnom Penh. We found that sediment inflow into the
Tonle Sap Lake was slightly lower than the sediment outflow
from the lake. The annual sediment load from Tonle Sap River
contributing to the Mekong delta is rather low when compared
to the Mekong mainstream. Its contribution during the dry
season is, however, important. Since the Mekong sediment dynamics have considerable influence on the Tonle Sap sediment
dynamics, the possible impacts of emerging hydropower dams
along the Mekong mainstream may threaten the sediment
transport into the floodplains. This would result in reduction
of the nutrient-rich sediment load and would therefore have a
direct impact on productive ecosystem functions. Therefore, a
detailed long-term sediment monitoring programme should be
undertaken in order to support sustainable development of
the Mekong River Basin.
Acknowledgements—This work was supported by the Ministry of
Education (MOE), Singapore (Grant Number R-109-000-086-646) and
Yunnan Province. Matti Kummu was further supported by the Aalto
University Postdoctoral Grant, the Academy of Finland project SCART
(grant no. 267463) and Maa- ja vesitekniikan tuki ry. The staff from
Resource Development International, Cambodia (RDI) is greatly
acknowledged for conducting the field measurements and sample
handling. The authors are grateful for the invaluable help and support
of Dr Erland Jensen, Dr Tes Sopharith, Dr Paradis Someth, Dr Heiko
Apel and Nguyen Van Manh. The constructive comments from the
editor and two reviewers are much appreciated.
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Supporting Information
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Earth Surf. Process. Landforms, Vol. 39, 1855–1865 (2014)