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Proc. IAHS, 383, 367–374, 2020
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Hydrological processes and water security in a changing world
Impact of hydropower dam on total suspended
sediment and total organic nitrogen fluxes of
the Red River (Vietnam)
Nhu Da Le1,2, Thi Phuong Quynh Le1,2 , Thi Xuan Binh Phung3, Thi Thuy Duong4, and Orange Didier5
1Graduate University of Science and Technology, 18 Hoang Quoc Viet road,
Cau Giay district, Hanoi city, Vietnam
2Institute of Natural Product Chemistry, Vietnam Academy of Science and Technology,
18 Hoang Quoc Viet road, Cau Giay district, Hanoi city, Vietnam
3Electric Power University, 235 Hoang Quoc Viet, Bac Tu Liem, Hanoi, Vietnam
4Institute of Environmental Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet
road, Cau Giay district, Hanoi city, Vietnam
5IRD, ECO&SOLS, University of Montpellier, CIRAD, INRA, Supagro Montpellier, Place Viala, 34060
Montpellier CEDEX 2, France
Correspondence: Le Thi Phuong Quynh (quynhltp@gmail.com, quynhltp@yahoo.com)
and Le Nhu Da (dalenhu@gmail.com)
Published: 16 September 2020
Abstract. The Red River is a typical example of the Southeast Asian rivers, which has been strongly affected by
human activities. This paper analyses the change of total suspended sediment (TSS) load of the Red River from
1960 to 2015 in which numerous new dams in both China and Vietnam have been constructed. A strong decrease
of TSS load of the whole Red River (from 79±26 ×106t yr−1in 1960s to 6 ±1×106t yr−1in 2010s) allocated
to the dam impoundments in spite of population and deforestation increase. Base on the experimental equation
describing the relationship between TSS and total organic nitrogen (TON) concentrations, and on the available
data of TSS concentration and river discharge, the longterm TON concentrations and fluxes were calculated
for the three tributaries and the whole Red River. The annual average of TON concentrations spatially varied
from 0.41 to 3.19 mg L−1, averaging 0.98 mg L−1for the whole period; the lowest was found for the Da River
where the new dams have been impounded. The highest TON concentrations and fluxes occurred in the wet
season in relationship with the highest sediment loads and river discharges. The riverine TON fluxes transferred
to estuary significantly decreased from 141 ×103±38 ×103t yr−1(equivalent to 902 ±247 kg km−2yr−1) in
1960s to 32 ×103±5×103t yr−1(equivalent to 207 ±35 kg km−2yr−1) in 2010s. The TSS flux decrease has
driven a clear reduction of associated elements like nitrogen, which let to hypothesis a change in biogeochemical
processes in the coastal zone.
1 Introduction
Total suspended solids (TSS) transported by the Asian rivers
to the coastal ocean play an important role in the global de-
livery of TSS and associated elements (C, N, P) due to the
significant quantities of suspended solids. However, in re-
cent decades, TSS and material fluxes of these rivers have
been altered drastically as a result of reservoir impound-
ment, land use, population, and climate change (Walling and
Fang, 2003). Within the factors affected to the riverine TSS
load, dams and reservoirs are known to be the most im-
portant. The impact of dam impoundment was clearly ob-
served for numerous Asian Rivers such as the Yangtze River
(Changjiang) (Wu et al., 2007), the Yellow River (Huanghe)
(in China) (Wang et al., 2007; Hu et al., 2015), the Mekong
River (Kummu and Varis, 2007)... Dam impoundment has
Published by Copernicus Publications on behalf of the International Association of Hydrological Sciences.
368 N. D. Le et al.: Impact of hydropower dam on total suspended sediment and total organic nitrogen fluxes
reduced TSS fluxes of these rivers by more than half. The
loss of sediment and associated nutrient fluxes from the con-
tinent has dramatically affected deltas (Syvitski and Saito,
2007). The habitats of many aquatic species in coastal zone
have significantly impacted the recovery of the mangroves
and coastal aquaculture area. Thus, Beusen et al. (2005) em-
phasized that it is important to understand the response of
riverine TSS and associated nutrient fluxes to regional and
global changes, and the main factors that control the fluvial
sediment transport to the oceans.
The Red River (Vietnam and China) is a typical Asian river
which is strongly impacted by both climate and human ac-
tivities. Indeed, human activities have altered the hydrology,
suspended sediment and associated element (C, N, P) loads
of the Red River though dam impoundment, land use change
and population increased (Le et al., 2015). Two large dams
(Hoa Binh, Thac Ba) have been impounded in the Red River
systems, in the Vietnamese territory since 1970s. And re-
cently, since the year 2010, new series of dams have been set
up in the Red River upstream in period 2000–2015 in both
China and Vietnam sectors. This study firstly aims to esti-
mate the recent change of TSS flux due to new series of dams
constructed in both China and Vietnam in the period 1960–
2015 basing a data set of daily TSS concentrations and daily
river discharge. And then, basing on the experimental equa-
tion describing the relationship between the TSS and total
organic nitrogen (TON) concentrations, and on the available
data of TSS concentrations and river discharges, the longterm
TON concentrations and fluxes were calculated for the three
tributaries and the whole Red River.
2 Site description and methodology
2.1 Site description
2.1.1 The Red River basin
The Red River situates in South-East Asia, drains a water-
shed area of 156 451 km2, 50.3 % of which is situated in Viet-
nam, 48.8 % in China and 0.9 % in Laos. The Red River in-
cludes three tributaries Thao, Da, Lo Rivers which join at
Viet Tri city, and then forms a large delta before flowing into
the South-East Asian Sea through four distributaries called,
Ba Lat, Lach Gia, Tra Ly and Day (Dang, 2000) (Fig. 1).
There are two distinct seasons in the Red River basin:
rainy season (May–October) and dry season (November–
April). The annual rainfall strongly varies from 700 to
4800 mm yr−1across the basin with about 80 % of rainfall
occurring during the rainy season. Noted that highest inci-
dence of typhoons occur in July and August in the basin area.
The three upstream sub-basins of Thao, Da and Lo rivers
have differences in their lithology. The Red main channel
basin is dominated by metamorphic rocks, except in the up-
per reaches where the Mesozoic sedimentary deposits are
found. The Da drainage basin is composed of sedimentary
rocks from the Mesozoic and Paleozoic period with minor
felsic intrusions. The Lo drainage has low-grade metamor-
phic rocks and Proterozoic to Paleozoic sedimentary rocks
with some granitic intrusions. In the central Red River Delta,
silt and clay are the most abundant sediments while fine
sand presents in small amounts from the Holocene succes-
sion (Moon et al., 2007).
Land use and population density are different for
the four sub-basins of the Red River. Low population
(<100 inhabitant km−2) and high proportion of forestland
are found for the Da river basin. The moderate popula-
tion and industrial-agricultural and forest land is observed
in the Lo and Thao river basins. Very high population
(>1000 inhabitant km−2) and high proportion of agricultural
land are characterized for the Delta area. Change in landuse
(deforestation and intensive agricultural land) and population
increased for longterm period from 1960s–2010s have been
revealed in a recent study (Le et al., 2015, 2018).
2.1.2 Hydrology and Reservoir impoundment
Hydrology
Daily river discharges at the outlets of the three tributaries
Thao (at Yen Bai station), Da (at Hoa Binh station) and Lo
(at Vu Quang station) and of the downstream main axe of
the Red River (at Hanoi station) were available for the pe-
riod 1960–2015 (MONRE, 1960–2015) (Fig. 1). The river
discharge follows the variation of rainfall during the year
with clear higher river discharges observed in the rainy sea-
son than in the dry one. The average river discharge of the
Thao (at Yen Bai), Da (at Hoa Binh), Lo (at Vu Quang)
and in the main axe at Hanoi stations were 720; 1670;
1020 and 2500 m3s−1respectively for the period 1960–2015
(MONRE, 1960–2015). Of the three upstream tributaries, the
Da River accounts for a half of the total Red River discharge.
Long-term data on the decade average of water discharge
showed a clear decrease for the Thao, Da tributaries and for
the main downstream Red River in the 1960–2015 period
(Table 1). During the whole study period, extreme low river
discharges of the main axe Red River at Son Tay (368 m3s−1
in May 1960 or 380 m3s−1in March 2002) hydrological sta-
tions were observed (Quach, 2011; Pham et al., 2015) and
extreme high river discharge (37800 m3s−1) at Son Tay sta-
tion was observed in August 1971.
Dam impoundment
In the Red River basin in both China and Vietnam territory,
a series of reservoirs and dams have been constructed and
operated (Ha and Vu, 2012; Le et al., 2018) for multipur-
pose including generating hydropower, controlling floods,
providing water for the agricultural land, regulating water
resource during dry season for downstream. In the upstream
Thao River, two main intercepting dams (namely Namsha
and Madushan, 140 km from the Vietnamese border to
Proc. IAHS, 383, 367–374, 2020 https://doi.org/10.5194/piahs-383-367-2020
N. D. Le et al.: Impact of hydropower dam on total suspended sediment and total organic nitrogen fluxes 369
Figure 1. The Red River and its tributaries: location of the large reservoirs and hydrological stations.
Table 1. Decade average and standard deviation (SD) of river discharge, TSS and TON concentrations (mg L−1) of the Thao, Da and Lo
tributaries and of the main axis of the Red River at Hanoi station in different periods from 1960 to 2015.
Hydrological
station River Variables 1960–1969 1970–1979 1980–1989 1990–1999 2000–2009 2010–2015
Yen Bai Thao River discharge, m3s−1774 ±126 818 ±208 674 ±103 752 ±137 708 ±140 517 ±34
TSS concentration, mg L−1883±274 965 ±79 1054 ±454 1479 ±344 928 ±297 278 ±62
TON concentration, mg L−11.5 ±0.4 1.7 ±0.1 1.8±0.6 2.3 ±0.4 1.6 ±0.4 0.7 ±0.1
Hoa Binh Da River discharge, m3s−11720 ±219 1706±213 1544 ±236 1840 ±301 1798 ±230 1276 ±216
TSS concentration, mg L−1475±144 477 ±75 354 ±169 74 ±15 56 ±26 15 ±6
TON concentration, mg L−11.0 ±0.2 1.0 ±0.1 0.9 ±0.2 0.5 ±0.0 0.5 ±0.0 0.4 ±0.0
Vu Quang Lo River discharge, m3s−1955 ±144 1077±227 1085 ±165 1098 ±158 897 ±213 990 ±439
TSS concentration, mg L−1158±28 167 ±54 163 ±56 207 ±51 140 ±53 56 ±22
TON concentration, mg L−10.6 ±0.0 0.6 ±0.1 0.6 ±0.1 0.7 ±0.1 0.6 ±0.1 0.5 ±0.0
Hanoi Main down- River discharge, m3s−12610 ±317 2821±380 2607 ±439 2660 ±390 2310 ±310 1673 ±262
stream Red TSS concentration, mg L−1505 ±112 554 ±105 434 ±116 389 ±74 343 ±172 77 ±8
River branch TON concentration, mgL−11.1 ±0.1 1.1 ±0.1 1.0 ±0.2 0.9 ±0.1 0.8 ±0.2 0.5 ±0.0
China) have been constructed in the period 2010s (IMRR,
2010). In the Lo River, the Thac Ba was constructed the
earliest, in 1972 originally designed to supply water and
generate hydropower (440 MW) but did not control floods.
Recently, the Tuyen Quang is impounded in the Vietnamese
part and at least 8 hydropower reservoirs have been built
on the upstream Lo River in the Chinese part (Ha and
Vu, 2012). In the Da River, a series of dams/reservoirs
have been constructed since 1989. Besides the 3 large
reservoirs (Hoa Binh, Son La and Lai Chau), small hydro-
electric power stations have been set up (Ban Chat, Huoi
Quang) on the Nam Mu river, a branch of the Da River
in Vietnam and about 11 small hydrological dams con-
structed in the Chinese part since 2007 (Ha and Vu, 2012).
2.2 Methodology
2.2.1 Data collection
Daily TSS concentrations and the daily river discharge data
of the 3 main tributaries: Yen Bai station (outlet of the Thao
river), Hoa Binh station (outlet of the Da river), Vu Quang
station (outlet of the Lo river), and in the main axe (at Hanoi
station) of the Red River (Fig. 1, Table 1) from 1960 to
2015 were obtained from the MONRE (1960–2015). Daily
TSS concentrations were determined at each gauging sta-
https://doi.org/10.5194/piahs-383-367-2020 Proc. IAHS, 383, 367–374, 2020
370 N. D. Le et al.: Impact of hydropower dam on total suspended sediment and total organic nitrogen fluxes
tion by the Vietnamese standard TCVN 6625:2000 method
(MOSTE, 2000).
2.2.2 Calculation of TSS fluxes
Daily TSS flux at the outlet of each tributary (Thao, Da
and Lo) and at Hanoi station (main downstream axe of the
Red River) was calculated based on the daily TSS concentra-
tions with the daily river discharge. Annual TSS flux export
(106t yr−1) at the outlet of the Thao, Da and Lo and at Hanoi
was calculated as the sum of daily export at each tributary
and at Hanoi over the course of a year. The total TSS fluxes
of the whole Red River were calculated by extrapolating the
flux measured at the Hanoi station taking into account the
respective Delta area in the watershed.
2.2.3 Calculation of TON concentrations and fluxes
Previous studies revealed that riverine particulate nitrogen
loads are strongly correlated with TSS loads (Ittekkot and
Zhang, 1989; Ludwig and Probst, 1996). Our previous study
(Le et al., 2005) demonstrated a relationship between TSS
concentrations and Total organic nitrogen (TON) concentra-
tions for the Red River water, basing on field measurements,
as shown in Eq. (1):
TON (mg L−1)=0.4+0.0013TSS (mg L−1)
(r2=0.91) (1)
Recently, we have analysed about 20 Red River samples
taken in Hanoi hydrological station for testing this equation
and found the possibility for application of this equation.
Thus, we use this equation for calculating daily TON con-
centrations at different stations Yen Bai (Thao River), Hoa
Binh (Da River), Vu Quang (Lo River) and Hanoi in the
main branch Red River for the period 1960–2015 basing on
the available daily TSS concentrations at the respective sta-
tions in this period. After obtaining daily TON concentra-
tions, daily and annual TON fluxes were calculated by the
same method for TSS flux calculation.
3 Results and discussion
3.1 Total suspended solids (TSS) and total organic
nitrogen (TON) concentrations
3.1.1 Spatial and temporal variation
TSS concentrations which varied 8 to 2144 mg L−1showed
spatially different variation for the three tributaries and of
the main branch of the Red River (at Hanoi stations). The
highest TSS was observed at Yen Bai station (Thao River)
in 1986 when the big flood occurred in this river whereas
the lowest value was found at Hoa Binh station (Da River)
in 2015 after the Son La dam has been in operation. For the
whole period 1960–2015, within the three upstream tribu-
taries, the mean TSS concentrations were the highest at the
Thao station (966 ±432 mg L−1) where less dams were im-
pounded in its main course. The TSS concentrations in the
Lo and Da Rivers were much lower and averaged 157 ±59
and 297 ±554 mg L−1, respectively. The TSS concentration
of main axe Red River at Hanoi station, averaging 411 ±
170 mg L−1over the period 1960–2015 was lower than the
one of the Thao River due to the dilution process because of
higher discharges and lower TSS concentrations of the Da
and Lo Rivers than those of the Thao River (Table 1).
Regarding temporal variation, the TSS concentrations at
all sites observed showed a clear decrease over the 1960–
2015 period. Within the three tributaries, a significant de-
crease of TSS concentration was found for the Da River
(from 475±144 mg L−1in 1960s to 15±6 mg L−1in 2010s)
where a series of dams were operated in its main course
(Table 1). Presently (2010–2015 period), within three main
tributaries, the TSS concentration of the Da river is lowest
(Table 1). The Thao and the Lo showed a clear reduction
of TSS concentrations (from 883 ±274 mg L−1in 1960s to
278 ±62 mg L−1in 2010s) and (from 158 ±28 mg L−1in
1960s to 56 ±22 mg L−1in 2010s, respectively) (Table 1,
Fig. 2), especially in the last decade where dams have been
impounded in the Thao and Lo rivers.
TON concentration followed the same spatial variation of
the TSS, being highest for the Thao River (1.7±0.6 mg L−1)
and lowest for the Lo River (0.6±0.1 mg L−1) over the 1960–
2015 period. A reduction of TON concentrations was also
observed at different stations, however, much lower in com-
paring with TSS concentration decrease. Indeed, TON con-
centrations were reduced by a haft at all sites observed (Ta-
ble 1). For the whole period, the annual average of TON con-
centrations spatially fluctuated from 0.41 to 3.19 mg L−1, av-
eraging 0.98 mg L−1. Presently (2010–2015 period), within
three main tributaries, the TON concentration of the Da river
was lowest (Table 1).
3.1.2 Seasonal variation
The TSS concentrations were higher in the rainy season than
in dry season at all four stations during all periods observed.
Indeed, over the whole period, the average TSS concentra-
tions in the Thao, Lo and Da Rivers during the rainy season
averaging 1495 ±744, 271 ±112 and 83 ±262 mg L−1re-
spectively were 5 times higher than the ones in dry season
averaging 301 ±159, 35 ±18 and 24 ±48 mg L−1respec-
tively. In the main branch of the Red River, at Hanoi station,
TSS concentrations were 3 times higher in the rainy season
(655±276 mg L−1) than in the dry season (171±90 mg L−1).
As known, most of riverine TSS fluxes of the tropical river
systems have transported during the rainy season, mainly at
high river discharge (Van Maren and Hoekstra, 2004). Higher
rainfall in rainy season accelerates soil erosion and leaching,
Proc. IAHS, 383, 367–374, 2020 https://doi.org/10.5194/piahs-383-367-2020
N. D. Le et al.: Impact of hydropower dam on total suspended sediment and total organic nitrogen fluxes 371
Figure 2. Annual average of TSS, TON concentrations of the Thao, Da and Lo Rivers and of the main axe at Hanoi station during different
periods from 1960 to 2015.
Table 2. Annual average and standard deviation of TSS fluxes (106t yr−1) and TON flux (103t yr−1) of the Thao, Da and Lo tributaries and
of the whole Red River system during different periods from 1960 to 2015.
Period 1960–1969 1970–1979 1980–1989 1990–1999 2000–2009 2010–2015
Thao (YB) TSS 34 ±18 38 ±12 36 ±25 59 ±23 41 ±21 7 ±1
TON 58 ±25 59 ±17 55 ±33 86 ±30 62 ±28 16 ±2
Da (HB) TSS 57 ±24 53 ±16 32 ±19 7 ±2 5 ±3 1 ±0
TON 95 ±33 90 ±23 61 ±27 32 ±6 29 ±6 17 ±3
Lo (VQ) TSS 8 ±3 10 ±4 9 ±4 12 ±3 8 ±5 2 ±1
TON 23 ±5 27 ±8 25 ±6 30 ±5 22 ±9 15 ±7
Whole RR TSS 79 ±26 86 ±22 63 ±24 58 ±17 44 ±25 6 ±1
TON 141 ±38 154 ±33 121 ±36 114 ±26 92 ±36 32 ±5
leading to higher riverine TSS concentrations in tropical re-
gion.
Associated with TSS, TON concentrations at all sites were
higher in the rainy season than in the dry season. Indeed, over
the whole period, the average TSS concentrations during the
rainy season in the three tributaries Thao (2.5±1.0 mg L−1) ,
Lo (0.7±0.1 mg L−1) and Da (0.6±0.3 mg L−1) Rivers and
the main axe at Hanoi station (1.2±0.4 mg L−1) were about
1.5–3.0 times higher than the ones in dry season (0.8±0.2,
0.5±0.1; 0.5±0.1 and 0.6±0.1 mg L−1respectively.
3.1.3 Total suspended solids (TSS) and total organic
nitrogen (TON) fluxes
Our calculations showed that the TSS fluxes of the Da River
at the Hoa Binh hydrological station decreased significantly
from 57 ×106±22 ×106t yr−1for the period 1960–1969 to
1.0×106±0×106t yr−1for the period 2000–2015 (Table 2).
Comparing with the period 1960–1969 (before the Hoa
Binh reservoir presence), the TSS fluxes of the Da River
in the period 1990s decreased about 87.8 %. Since the addi-
tional presence of the Son La, Lai Chau, Huoi Quang reser-
voirs in Vietnam and 11 small hydroelectric powers in China,
the TSS fluxes at the outlet of the Da River further decreased
at the total rate of 98.6 %. It clearly shows the important role
of the reservoirs in TSS storage, leading to a clearly decrease
of TSS fluxes at the outlet of the Da River (Table 2). For the
Lo and the Thao Rivers, similarly, in the period 2000–2015,
the TSS fluxes at the outlets of these rivers decreased by 72%
and 80 % respectively, comparing to the pre-construction the
hydropower plants in both Vietnam and China (Table 2).
Previous study found that about 70 % reduction of sedi-
ment flux was observed for the Red River after the impound-
ment of the Hoa Binh and Thac Ba reservoirs on the Da and
Lo rivers (Le et al., 2007; Dang et al., 2010). Our study re-
vealed that actually, at Hanoi site, the TSS fluxes in the pe-
riod 2010–2015 decreased by 92 % in comparing with the pe-
riod without reservoir impoundment in the whole Red River
system (1960s) (Table 2). In addition, at Hanoi station (dow-
stream of the Red River), we observed that the sum of the
annual TSS fluxes of the three tributaries Thao, Da and Lo
(input to the Delta area) was higher than the mean annual
fluxes of the Hanoi site (output). This is due to the complex-
ity of the hydrological network in the Delta area where some
distributaries flow out from the main branch of the Red River
(Duong, Day and Nhue Rivers) (Luu et al., 2010) and the im-
portant deposition process in the section from the Viet Tri –
Son Tay – Hanoi, the floodplains (Lu et al., 2015).
Overall, the TSS fluxes of the whole Red River trans-
ferred to the estuary significantly decreased from 79 ×106±
26 ×106t yr−1(equivalent to a specific sediment yield of
505 ±166 t km−2yr−1) in the period 1960s to 6 ×106±1×
https://doi.org/10.5194/piahs-383-367-2020 Proc. IAHS, 383, 367–374, 2020
372 N. D. Le et al.: Impact of hydropower dam on total suspended sediment and total organic nitrogen fluxes
106t yr−1(corresponding to a specific sediment yield of
38 ±6 t km−2yr−1) in 2010–2015 period.
The TSS fluxes of the Red River at present situation
(2010s) were much lower than that of the large Asian Rivers
such as the Ayeyarwady (1966 to 1996): 325×106t yr−1(Fu-
ruichi et al., 2009).
In Asia, reservoirs have been constructed on large rivers
for multi-purposes, including hydropower, flood control/flow
management, irrigation/water supply. . . The dam/reservoir
impoundment resulted in dramatically reducing of sediment
flux discharged to the sea which was observed for numer-
ous Asian rivers. For example, the sediment discharge of the
Yangtze River (Changjiang) decreased from 480 ×106t yr−1
to approximately 150 ×106t yr−1over a 20-year period
(Wang et al., 2008) whereas the Yellow River (Huanghe) (in
China) reduced from 1080 ×106to 150 ×106t yr−1over a
40-year period (Wang et al., 2007). For some cases of Asian
river, e.g the Minjiang River (Xu and Yan, 2010) and the Up-
per Mekong River (Kummu and Varis, 2007), different reser-
voirs were built in different times, and thus, the sediment
trapping induced by different reservoirs has various phases,
driven to the complicated effects caused by a single reservoir.
In addition, associated with riverine TSS, the reduction of
TSS flux could decrease a large particulate nutrients (N, P)
and carbon load to the estuary. In this study, TON load of the
whole Red River reduced from 141 ×103±38 ×103t yr−1
(equivalent to a specific yield of 902±247 kg km−2yr−1) in
the period 1960s to 32×103±5×103t yr−1(corresponding to
a specific sediment yield of 207±35 kg km−2yr−1) in 2010–
2015 period.
The construction of reservoirs has caused sudden loss of
huge amounts of fresh water, sediment and nutrients from the
rivers into the estuaries and coastal areas. Some major river
systems in Asia, such as the Yellow River, the Changjiang
River, the Mekong River, . . . have significant decreases in
water flow and suspended solid load due to environmental
changes, especially the human impacts in recent decades (Lu
and Siew, 2006; Kummu and Varis, 2007; Wang et al., 2012;
Lu et al., 2015). The decreases of suspended solids and as-
sociated substances (C, N, P and Si) loads can have serious
consequences, such as increasing coastal erosion, reducing
nutrient elements which provide foods for phytoplankton and
aquaculture, decreasing aquaculture production, loss of shel-
ter and breeding grounds. . . in coastal zone, as observed for
numerous Asian rivers (Chen, 2000).
The same is likely to be true for the Red River where the
impoundment of the large dams resulted in a clear decrease
of both TSS concentration and fluxes in this study. Noted that
a clear population and deforestation increases were observed
for the Red River basin in the period 1960s–2010s (Le et al.,
2015). This may approve the important role of dam impound-
ment in reducing suspended solids and associated nutrients
transfer of the in the Red River to the coastal zone. The de-
crease of TSS and TON fluxes of the Red River may give
different sequences (increased erosion, salinization of culti-
vated land or damaged ecosystems) in the coastal zone.
3.2 Bias in our calculation
Our calculations of TON concentration and flux of the Red
River are subject to a number of possible biases when the
TON concentrations were extrapolated from the relationship
between TON and TSS concentrations given in (Le et al.,
2005) with daily TSS concentrations obtained over the period
1960–2015. The use of this equation may not be suitable for
the entire longterm period (about 55 years for the Red River),
however, this bias is difficult for evaluating when no contem-
poraneous observation data exist. In addition, nitrogen trans-
formation and lost towards the atmosphere as N2was not
considered in our calculation of the Red River TON fluxes
transported to the coastal zone. However, despite these po-
tential errors, the results presented here show the importance
of the change in the Red River TON flux over the longterm
period 1960–2015.
4 Conclusion
Longterm variation of TSS and TON concentrations and
fluxes of the Red River were calculated for the period 1960–
2015. The results showed that a clear decrease of both TSS
and TON concentrations and fluxes of the three main tribu-
taries and the whole Red River in this period due to a series
of dams constructed in both China and Vietnam, especially
for a recent period 2010–2015, in spite of population and de-
forestation increase in the whole basin.
For detail, spatial and seasonal variations of TSS and TON
concentrations and fluxes of the Red River were clearly ob-
served. The annual average of TSS concentrations spatially
varied from 15 ±5 to 1479 ±326 whereas the annual aver-
age of TON concentrations spatially fluctuated from 0.41 to
3.19 mg L−1, averaging 0.98 mg L−1for the whole period.
Presently, the lowest values of both TON and TSS were de-
tected for the Da River where several new dams (including
the Son La and Lai Chau) appeared in the last decade 2010s.
The higher TSS and TON concentrations were found in wet
than in dry season for all sites during different periods.
Overall, the TSS fluxes of the Red River transferred
to the estuary significantly decreased from 79 ×106±
26 ×106t yr−1(equivalent to a specific sediment yield of
505 ±166 t km−2yr−1) in the period 1960s to 6 ×106±1×
106t yr−1(corresponding to a specific sediment yield of 38±
6 t km−2yr−1) in 2010s. Similarly, the riverine TON fluxes
transferred to estuary significantly decreased from 141 ×
103±38×103t yr−1(equivalent to 902±247 kg km−2yr−1)
in 1960s to 32 ×103±5×103t yr−1(equivalent to 207 ±
35 kg km−2yr−1) in 2010s. A strong decrease of TSS and
TON fluxes of the whole Red River may give different se-
quences (increased erosion, salinization of cultivated land or
Proc. IAHS, 383, 367–374, 2020 https://doi.org/10.5194/piahs-383-367-2020
N. D. Le et al.: Impact of hydropower dam on total suspended sediment and total organic nitrogen fluxes 373
damaged ecosystems) in the coastal zone of the Red River,
as observed for numerous Asian rivers.
Data availability. Data is not publicly accessible.
Author contributions. LND and PTXB calculated TON fluxes.
LND and LTPQ prepared the manuscript with contributions from
all co-authors.
Competing interests. The authors declare that they have no con-
flict of interest.
Special issue statement. This article is part of the special issue
“Hydrological processes and water security in a changing world”.
It is a result of the 8th Global FRIEND–Water Conference: Hydro-
logical Processes and Water Security in a Changing World, Beijing,
China, 6–9 November 2018.
Financial support. This research has been supported by the
Graduate University of Science and Technology /Ministry of Indus-
try and Trade of the Socialist Republic of Vietnam (GUST.STS.ÐT
2020-MT01 /DTKHCN.008/19 project).
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Proc. IAHS, 383, 367–374, 2020 https://doi.org/10.5194/piahs-383-367-2020
