water

Article Water and Suspended Sediment Budgets in the Lower Mekong from High-Frequency Measurements (2009–2016)

Dang Thi Ha 1, Sylvain Ouillon 2,3,* ID and Giap Van Vinh 4

1 School of Maritime Economics and Technology, Ba Ria-Vung Tau University, 84254 Ba Ria-Vung Tau, Vietnam; [email protected] 2 LEGOS, Univ. Toulouse, IRD, CNRS, CNES, 14 av. E.-Belin, 31400 Toulouse, France 3 Department of Water Environment Oceanography, University of Science and Technology of Hanoi (USTH), 18 Hoang Quoc Viet, 100000 Hanoi, Vietnam 4 Cuu Long River Hydrological Center, Southern Regional Hydro-Meteorological Center (SRHMC), 84282 Can Tho, Vietnam; [email protected] * Correspondence: [email protected]; Tel.: +33-561332935



Received: 19 April 2018; Accepted: 20 June 2018; Published: 26 June 2018


Abstract: Based on a new dataset of high temporal resolution of water discharge (hourly frequency) and suspended sediment concentration (twice daily frequency at ebb and flood tides) at Can Tho and My Thuan stations during the 2009–2016 period, monthly and annual flow and suspended sediment flux of the lower Mekong River were calculated. The present water discharge of the Mekong River to the sea can be estimated to be 400 km3 yr−1, +/− 100 km3 yr−1 depending on El Niño Southern Oscillation (ENSO), and the present sediment supply to the sea can be estimated to be 40 Mt yr−1, +/− 20 Mt yr−1 depending on ENSO. The ENSO influence (proxied by the Southern Oscillation Index—SOI) on Q (water discharge) and Qs (sediment flux) is at maximum at a time lag of 8–9 months. The 2010–2011 La Niña event increased the water supply by almost 30% and the sediment supply by 55% in 2011. The 2015–2016 El Niño event decreased the water supply by 20% and the sediment supply by 50% in 2015–2016. The average net water discharge was 12,550 m3 s−1 in neutral years. The Tien River at My Thuan accounted for ~52%, and the Hau River at Can Tho for ~48% of the total Mekong River discharge, with small variations (of the order 1%) amongst years. In terms of suspended sediment delivery to the sea, the Tien River accounted for a more variable portion of 55% in neutral years, 57.3% in 2015–2016 (El Niño) and 62% in 2011 (La Niña). 80% of water discharge occurred during the flood season (July–December) and 20% occurred in the low flow season (January–June), the proportion being 91% and 9% for the sediment supply. Previous estimates of annual sediment supplies to the sea (145–160 Mt yr−1) had been established before 2003. Possible origins of the recent reduction by ~75% of sediment supply are presented and discussed.

Keywords: Mekong; river discharge; suspended sediment; dams; Tien River; Hau River; ENSO; El Niño; La Niña

1. Introduction Weathering and mechanical erosion, which constrain the fluxes of sediment and associated elements (e.g., trace elements, nutrients, organic matter) from the continent to the ocean, are attributed to a combination of natural processes related to geomorphology, tectonic activity, and climate influences as well as land cover [1–6]. Additionally, human disturbance affects erosion and sediment supply to estuaries, deltas or coastal zones [7–10]. Recently, there has been increasing concern about the global river sediment balance, particularly with respect to its response to climate change and anthropogenic

Water 2018, 10, 846; doi:10.3390/w10070846 www.mdpi.com/journal/water Water 2018, 10, 846 2 of 24 impacts [6,7,11–16]. However, important uncertainties persist, mainly due to the nonstationary nature of sediment fluxes and the use of dated and/or questionable data (short-term sampling, inappropriate sampling frequency and/or data collected before dam/reservoir construction, deforestation or climatic changes [9,17–19]). The rivers draining the Himalaya Mountains into South-East Asia have been recognized as significantly contributing to suspended particulate matter (SPM) delivery to the global ocean [4,5,13,20]. In this key area, recent and rapid changes in population and economic growth have strongly affected the functioning of the river-systems leading to major human pressure on its sustainable development. Therefore, South-East Asian Rivers are good indicators of the strong influence of anthropogenic activities on suspended sediment transport. The sediment supply has declined in many large rivers with the construction of large reservoirs: in the Yellow River [14,21]; in the Changjiang (Yangtze) River [14,22–24]; in the Red River [25,26], as previously observed on others large rivers such as in the Nile River [4] and the Mississippi River [21]. Monitoring and predicting the modification of sediment fluxes in South-East Asia represents one of today’s major challenges for environmental sciences, because of the continually shifting anthropogenic and/or climatic impact and the need for continuous updated fluvial data for management purpose. The Mekong River is the second largest river basin (795 × 103 km2) after the Yangtze River (1.8 × 106 km2) and the third in terms of water discharge (after the Ganges-Brahmaputra and the Yantgze Rivers) in Southeast Asia. The Mekong basin is shared by six riparian countries: China, Myanmar, Laos, Thailand, Cambodia, and Vietnam. In recent years, the basin has faced rapid development related to water resources management, including various hydropower plans and large irrigation schemes [27,28]. Reservoir operation and climate change are among the most influential drivers of future hydrological change in the Mekong, and other drivers include land cover change, new irrigation and water diversion schemes, and urbanization [29,30]. The impact of reservoir operation on the Mekong basin has been studied by different actors, including the Mekong River Commission (MRC), the Asian Development Bank (ADB), the United Nations Education Scientific and Cultural Organization (UNESCO), the National Natural Science Foundation of China (NNSFC), the World Wild Fund (WWF), the US National Science Foundation (NSF) and the Office of Naval Research (ONR) (e.g., [31–46]), showing that the reservoirs in the upper Mekong basin have trapped most of the sediment from the upper reaches, and the hydrological regime and river direction was profoundly changed due to water regulation, with lower flood peaks and higher dry season flows. However, most analysis were based on “recycled” database (e.g., [18,47] and/or used inadequate sampling frequencies, including monthly to weekly sampling (e.g., [34,38,41]). The serious lack of long-term and accurate sediment data in the Mekong River makes the sediment flux into the estuary impenetrable [18,39,42,48,49]. The Research Project “The erosion process in the Lower Mekong Delta Coastal Zones (LMDCZ) and the measures for protecting Go-Cong and U-Minh from coastal erosion” supported by European Union (EU), Agence Française de Développement (AFD) and Southern Institute of Water Resources Research (SIWRR) aims at understanding the mechanism of the erosion/accretion process by identifying the main causes responsible for this erosion, and then setting up measures in order to protect Go-Cong and U-Minh from coastal erosion. Within this project, a budget of sediment supply by the Mekong River in its delta was requested. The study was based on the most recent extensive dataset of hourly water discharges and twice-daily suspended sediment concentrations (SSC), collected between 2009 and 2016 at 2 strategic sites (Can Tho and My Thuan gauging stations). Before 2009, no regular accurate data on water discharge and sediment concentration was available on the lower Mekong River (including Thailand, Cambodia and Vietnam areas). The present paper summarizes the main information derived from this dataset. Its objectives are to quantify the actual water and sediment flux by the Mekong River to the sea and analyze the spatial-temporal variation of sediment flux to the sea of the Mekong River, at short term with the new dataset (2009–2016), and over a longer perspective using additional data from the literature. Water 2018, 10, 846 3 of 24 Water 2018, 10, x FOR PEER REVIEW 3 of 24

2.2. MaterialsMaterials andand MethodsMethods

2.1.2.1. StudyStudy AreaArea TheThe MekongMekong River River is theis the 10th 10th longest longest river river in the in world the withworld a totalwith length a total of 4909length km. of The 4909 Mekong km. RiverThe Mekong originates River from originates the Tibetan from the plateau Tibetan and plateau flows throughand flows a through narrow a deep narrow gorge deep along gorge with along the Salweenwith the andSalween Yangtze and Yangtze Rivers that Rivers together that together are known are known as the ‘Threeas the ‘Three Rivers Rivers Area’. Area’. The Mekong The Mekong River thenRiver flows then throughflows through Myanmar, Myanmar, Laos, Laos, Thailand, Thailand, and Cambodiaand Cambodia before before it finally it finally drains drains into into the the sea creatingsea creating a large a large delta delta in Vietin Viet Nam Nam (Figure (Figure1). 1). The The Mekong Mekong flowsflows inin the delta through through two two main main distributaries:distributaries: thethe Tien Tien River River (generally (generally referred referred to to as as the the Mekong Mekong River, River, in thein the delta) delta) in itsin easternits eastern part, whichpart, which accounts accounts for nearly for 58%nearly of the58% total of Mekongthe total waterMekong discharge, water discharge, and the Hau and River the (theHau “second River river”(the “second in Vietnamese, river” in also Vietnamese, known as also Bassac known River) as Bassac in its western River) in part, its western which accounts part, which for 42%accounts of the Mekongfor 42% of discharge the Mekong [50]. discharge Downstream [50]. Downstream to My Thuan, to theMy TienThuan, River the Tien separates River intoseparates other into important other distributaries:important distributaries: The Co Chien The River,Co Chien the HamRiver,Luong, the Ham the Luong, Dai and the the Dai Tieu and Rivers, the Tieu from Rivers, Southwest from toSouthwest Northeast. to Northeast.

Hydrological station Tien River My Thuan Hau R.

Can Tho

Figure 1. Hydrological network of the Mekong River Basin. Figure 1. Hydrological network of the Mekong River Basin. The river discharge of the Mekong is mostly controlled by the tropical monsoon climate, which has distinctThe river wet discharge and dry of seasons the Mekong [51]. Mean is mostly annual controlled rainfall byvaries the tropicalfrom 1000 monsoon mm in climate,Thailand which to 3200 has distinctmm in Laos wet and[52]. dry In the seasons Mekong [51]. Ri Meanver basin, annual 85% rainfall of water varies discharge from 1000 occurs mm during in Thailand the flood to 3200 season mm inand Laos 15% [52 occurs]. In the in the Mekong low flow River season basin, [53]. 85% The of di waterscharge discharge is the largest occurs from during August–September the flood season and and

Water 2018, 10, 846 4 of 24

15% occurs in the low flow season [53]. The discharge is the largest from August–September and the smallest in April–May [54]. In contrast to other Asian rivers, the sediment discharge of the Mekong River remained constant on a millennial time scale, at a rate of 144 +/− 34 million tons per year since 3 kyr, which resulted in a constant expansion of the delta front [55]. The long term mean annual sediment load of the Mekong River at its mouth was estimated to be at 145–160 Mt yr−1 [5,56,57], and it seems from remotely sensed measurements of surface SPM concentrations that its flux has recently reduced [58]. Many studies have analyzed the impact of low-frequency climate variability on hydrological conditions in the Mekong River basin and the El Niño Southern Oscillation (ENSO) indices were widely used (e.g., [30,38,39,59–61]). They showed that the ENSO phases significantly influence precipitation, runoff and water level in the Mekong River basin. Piton and Delcroix [62], based on data from 43 consecutive years of in situ measurement (1960–2002) and seven years of satellite monitoring (1996–2002), demonstrated that El Niño phases were associated to a decrease in rainfall in the middle and lower Mekong basin and to a reduction of water discharge at the Chroy Chang Var station (located in Cambodia) with a lag of 6 months. The discharge reduction reached 34% during the strong 1997–98 El Niño. On the other hand, higher discharges were observed during La Niña events, with a similar lag. Hydropower is a key active sector in the Mekong River basin, particularly in China and Laos (Table 1;[63]). Total hydropower potential in the upper Mekong basin is estimated to be 23,000 MW, while total hydropower potential in the lower Mekong basin is 30,000 MW [37]. This includes 13,000 MW of 11 projects on the Mekong’s mainstream, and the remaining on its tributaries. The Manwan Reservoir, built in 1993 in China in the upper reach of the river, was the first large reservoir commissioned on the main stream of the Mekong River. About 11 projects are operating and under construction in China in the upper part. Recently, two large storage dams on the Mekong cascade (the 4200 MW Xiaowan and the 5850 MW Nuozhadu hydropower projects, with 9800 and 12,400 million m3 of active storage) started to be fully operating (Figure 1, Table 2). About 74 projects in the lower part are operating and 22 projects are under construction including Xayaburi (1260 MW) and Don Sahong (260 MW) mainstream projects (Table 1). Although the reservoirs in the upper Mekong basin have trapped most of the sediment from the upper reaches, little information is available on the impact of dams on the sediment load in the lower Mekong basin (especially in the Mekong Delta) and on the sediment flux by the Mekong River to the sea. Walling [18] estimated that if the major dams in the upper Mekong had little impact on the sediment load until 2002 (in a study covering 1960–2002), they likely may decrease the sediment load in a near future.

Table 1. Number of planned and proposed dams in the Mekong River basin (source: [63]).

Country Planned Dams Proposed Dams China 11 2 Laos 43 20 Myanmar 7 0 Thailand 7 0 Cambodia 12 0 Vietnam 1 0 Total 74 22

The sediment supply in the Mekong delta is impacted as well by other anthropogenic activities, such as groundwater pumping, irrigation, dredging, sand mining and land use changes (deforestation, soil conservation measures) [64–67]. The Mekong Delta of Vietnam is surrounded by: (a) Vietnam-Cambodia border in the North, (b) Pacific Ocean/South China Sea to the East (the so-called East sea), (c) Gulf of Thailand in the West (the so-called West sea), and (d) Vam Co Dong River and Ho Chi Minh City in the North-West. Water 2018, 10, 846 5 of 24

Hydrodynamic conditions in the Mekong Delta are affected directly by the river flow, the tidal regime of the East Sea (South China Sea) and for some parts of the delta by the tidal regime in the Gulf of Thailand (West Sea). The East Sea has a semi-diurnal and irregular sea-tide regime, while the West Sea is diurnal. Ogston et al. [68] show that reversing currents were measured until ~100 km landward in wet season and until ~190 km in dry season (their Figures 2 and 4). This mechanism has also been studied from numerical models (see, e.g., Figures 5 and 10 of Xing et al. [69]).

Table 2. List of recent or planned main hydropower dams in the main stream of the Mekong River (source: [63]).

Capacity Annual Generation Total Storage Dam Country Commissioning (MW) (GWh) (million m3) Manwan China 1500 7870 920 1986–1993 Dachaoshan China 1350 7090 880 1997–2002 Gongguquiao China 750 4670 510 2009–2012 Xiaowan China 4200 18,540 15,130 2010–2012 Jinhong China 1500 8470 1040 2012–2013 Nuozhadu China 5500 22,670 24,670 2013–2016 Xayaburi Laos 1295 6035 1300 2012–2020

2.2. Data In this study, the available dataset compiles hourly water discharge and twice-daily suspended sediment concentration at the Can Tho and My Thuan gauging stations from 2009 to 2016, supplied by the Cuu Long River Hydrological Center. My Thuan and Can Tho stations quantify flow of the Tien and Hau Rivers after flowing through the Plain of Reeds area (POR) and Long Xuyen Quadrant (LXQ), and after distribution of water through main irrigation channels in the POR and LXQ regions. My Thuan and Can Tho stations, which are influenced by sea tide, are located in the estuaries of these two main distributaries of the Mekong: Can Tho at 85 km from the mouth, and My Thuan at 100 km from the mouth. The observed alternation of seaward and landward currents in Can Tho and My Thuan was already described in the literature [50,68–72]. The water level is continuously recorded at the Can Tho and My Thuan gauging stations. Every hour, the flow has also been measured with a LS25 1A current meter at five depths in the main channel in front of these two stations from a metal boat, since 2009. Four times a year (in January, April, July and October), transects of currents are regularly measured by ADCP across the river, and currents are measured as well by a flow meter at 5 depths in the main channel. During these six-day calibration campaigns, about 35 transects are performed during ebb tides, and 35 during flood tides. These measurements enable to check the accordance of current measurements, and to establish rating curves between the flow depth profile from the flow meter in the main channel, the river discharge integrated over the cross-section and the water elevation. The rating curves, established four times a year, are used to infer river discharge from water level at hourly step [73]. All the year long, suspended sediment concentrations in the main channel (Cc) are determined twice a day (one at ebb tide, one at flood tide) from water sampling and weighting [74]. Water is sampled from the surface to the bottom using an integrating isokinematic-filling bottle. Sampling is performed at the time of the maximum velocity in the river. Water is filtered on “blue filters” (Chinese brand) with a 8 µm porosity. During the 70 ADCP transects of the four annual calibration field campaigns, water is also sampled along four profiles across the river to estimate the cross-averaged suspended sediment discharge Qs, and establish a rating curve between Cc and the “representative” suspended sediment concentration (Cav = Qs/Q) enabling to better estimate the sediment discharge. Two linear curves are established (Cav = k Cc + b), one for flood, one for ebb tide. A maximum of 15% of outliers are eliminated, and generally more than 75% of remaining points lie within the envelope +/− 15% of the rating curve. The resulting relationships Cc-Cav (one for ebb tide, one for flood tide) Water 2018, 10, 846 6 of 24 are used during three months to estimate the cross-section average suspended sediment concentration from the measured Cc. For the analysis of climatic variations, in particular to assess the impact of ENSO on the water and sediment supplies, different indexes were considered, provided by the National Oceanic and Atmospheric Administration (NOAA). The South Oscillation Index (SOI) presented in this paper was downloaded from [75]. To put our results in perspective over a longer time scale, data extracted from the literature were also considered; they are introduced in the discussion. It was hard to gather data at different stations (from upstream to downstream of the Mekong River basin) and for different periods (pre- and post-dams) with the same frequency sampling (e.g., daily measurements). Before 2009, programs for sediment and water discharge monitoring were discontinuous or had very low sampling frequency. Since 2009, the Mekong River basin has been well gauged, and the daily-hourly water discharge and suspended sediment concentration at major stations have been recorded (especially in the lower part). Thus, a more accurate annual sediment flux transported in the Mekong River can be calculated.

2.3. Data Analysis The sediment flux in ebb tide and flood tide conditions were calculated following:

Qs,e = Qe × Cav,e (1)

Qs,f = Qf × Cav,f (2) where Q stands for the water supply during an ebb or flood period, Qs for the suspended sediment supply during this period, Cav for the average suspended sediment concentration, and subscripts e and f stand for ebb and flood, respectively. In the data process, water discharges were calculated every 10 s by linear interpolation between the measurements performed at one-hour interval, so as to discriminate flows seaward and landward and estimate their budgets. −1 The monthly net sediment flux (Qs j, in Mt month ) of month j was then computed from the contributions of all ebb tides and flood tides of the month:

i=n i=n Qs j = ∑ Qs,e i − ∑ Qs,f i (3) i=1 i=1 where n stands for the number of days in month j, and the annual sediment flux (Qsa, in Mt) from:

12 Qsa = ∑ Qs j (4) j=1

The identification of breaking points during the period of study, if any, is based on cumulative discharge and sediment supplies [76,77]. The variability of time series (either monthly or yearly values) is assessed by the coefficient of variation (CV), defined by the ratio of the standard deviation to the mean (e.g., [78]). Monthly values of the Southern Oscillation Index (SOI) were considered to analyze the importance of the ENSO-related climate variability on monthly values of water discharge, sediment yield and sediment delivery to the ocean in the lower Mekong River.

3. Results

3.1. Monthly Averages and Seasonal Variations Monthly mean water discharges and suspended sediment fluxes of the Mekong River at Can Tho and My Thuan stations were calculated over the eight-year monitoring period (2009–2016) (Figure 2, Table 3). The hydrological variations at both gauging stations reflect the river’s response to seasonal Water 2018, 10, 846 7 of 24 rainfall and runoff: The dry season (November to April) is followed by a low flow season (January to June), whereas the rainy season (May to October) is characterized by high water discharges (July to November) (Figure 2, Table 3). Monthly mean water discharge in the Mekong River varied between 691 m3 s−1 to 18,647 m3 s−1 (temporal variation factor = 27) at Can Tho station, and between 834 m3 s−1 to 20,370 m3 s−1 (factor = 24) at My Thuan station. In the period 2009–2016, 82% of water discharge occured during the flood season (July–December) and 18% occurs in the low flow season (January–June) Waterat Can 2018 Tho,, 10, x the FOR proportion PEER REVIEW being 79% and 21% at My Thuan. The discharge was the largest7 fromof 24 August to October and the smallest in March–April (Table 3). The sediment supply occurred at sediment90% during supply the six-month occurred at flood 90% period during and the 10%six-month during flood the 6-month period and low 10% flow during period the at Can6-month Tho, lowthe proportionsflow period being at Can 92% Tho, and the 8% proportions at My Thuan being at the 92% same and periods. 8% at My In Thuan average at for the the same lower periods. Mekong, In average80% of water for the discharge lower Mekong, occurred 80% during of water the flood discharge season occurred and 20% during occurred the inflood the lowseason flow and season, 20% occurredthe proportion in the beinglow flow 91% season, and 9% the for proportion the sediment being supply. 91% and 9% for the sediment supply. The evolution ofof monthlymonthly mean mean sediment sediment flux flux is veryis very similar similar at both at both stations stations of the of Mekong the Mekong River, River,with high with seasonal high seasonal variations, variations, which are which constrained are constrained by the variability by the variability of water dischargeof water (Figuredischarge2). −1 (FigureThe monthly 2). The mean monthly sediment mean flux sediment ranged flux from ranged 0.01 Mtfrom month 0.01 −Mt1 (in month April 2010)(in April to 10.54 2010) Mt to month10.54 Mt−1 −1 −1 −1 month(in October (in October 2009); and 2009); from and 0.04 from Mt month0.04 Mt− 1month(in January (in January 2016) to 2016) 11.9 to Mt 11.9 month Mt month−1 (in October (in October 2011) 2011)at Can at Tho Can and Tho My and Thuan My Thuan stations, stations, respectively. respectively.

14 25000 Can Tho Monthly mean water discharge (m3/s) 12 (m Water discharge Monthly mean sediment flux (Mt/month) 20000 10

8 15000

6 10000 4 3

5000 /s) 2 Sediment Flux (Mt/month) 0 0 J-16 J-14 J-14 J-11 J-09 J-09 F-16 F-11 S-15 S-10 A-13 A-15 A-10 D-16 D-11 N-14 N-09 O-12 M-12 M-13 12 20000 My Thuan Monthly mean water discharge (m3/s) 10 Monthly mean sediment flux (Mt/month) (m Water discharge 15000 8

6 10000

4 3

5000 /s) 2 Sediment Flux (Mt/Month) 0 0 M-… M-… J-09 J-09 J-11 J-14 J-14 J-16 F-11 F-16 S-10 S-15 A-10 A-13 A-15 D-11 D-16 N-09 N-14 O-12

−1 3 −1 Figure 2. Evolution ofof monthlymonthly meanmean sedimentsediment fluxflux(Mt (Mt month month−1) andand waterwater dischargedischarge (m(m3 ss−1)) at CanTho ( upper panel) and MyThuan ( lower panel) stations during the 2009–2016 period.

Table 3. Monthly mean water discharge and sediment flux at Can Tho and My Thuan stations during the 2009–2016 period.

Can Tho Station My Thuan Station Month Q Qs Q Qs (m3 s−1) (Mt month−1) (m3 s−1) (Mt month−1) January 3359 0.382 3725 0.392 February 2136 0.238 2386 0.248 March 1330 0.141 1800 0.171 April 1251 0.114 1860 0.121 May 1689 0.157 2490 0.168 June 3360 0.530 4174 0.615 July 6358 1.478 7361 1.998

Water 2018, 10, 846 8 of 24

Table 3. Monthly mean water discharge and sediment flux at Can Tho and My Thuan stations during the 2009–2016 period.

Can Tho Station My Thuan Station Month Q Qs Q Qs (m3 s−1) (Mt month−1) (m3 s−1) (Mt month−1) January 3359 0.382 3725 0.392 February 2136 0.238 2386 0.248 March 1330 0.141 1800 0.171 April 1251 0.114 1860 0.121 May 1689 0.157 2490 0.168 June 3360 0.530 4174 0.615 July 6358 1.478 7361 1.998 August 10,296 2.688 11,194 3.636 September 12,502 3.900 12,687 4.802 October 13,270 3.988 13,414 5.378 November 9785 1.696 9853 2.507 December 5624 0.882 6085 1.234

3.2. Annual Average and Interannual Variations The annual water discharge and sediment flux of the Mekong River at Can Tho and My Thuan stations calculated from the database were compiled in Table 4, and the values for the whole Mekong River, estimated as the summation of the two contributes, are presented in Figure 3.

Table 4. Annual water discharge and sediment flux at Can Tho and My Thuan stations during the 2009–2016 period.

Can Tho Station My Thuan Station Year Q Annual Qs Annual Q Annual Qs Annual (m3 s−1) (Mt yr−1) (m3 s−1) (Mt yr−1) 2009 6381 34.22 6948 30.70 2010 4929 11.13 5410 14.65 2011 7660 23.37 8572 38.32 2012 5774 12.48 6363 17.24 2013 6582 15.02 6888 23.70 2014 6482 16.39 6791 22.70 2015 4347 8.43 4737 9.15 2016 5151 8.51 5645 13.63 Min 4347 8.43 4737 9.15 Max 7660 34.22 8572 38.32 CV (%) 18.1 54.1 18.4 42.4 Average 5913 16.20 6419 21.26

These results show the high temporal variations of annual sediment fluxes in the two main distributaries of the lower Mekong River (Table 4). The annual sediment fluxes ranged from 8.43 (in 2015) to 34.22 Mt yr−1 (in 2009) around the average value of 16.20 Mt yr−1 at the Can Tho station, with a coefficient of variation of 54%. For the My Thuan station, annual sediment fluxes ranged from 9.15 (in 2015) to 38.32 Mt yr−1 (in 2011) around the average value of 21.26 Mt yr−1, with a coefficient of variation of 42%. Assuming that the sum of sediment fluxes at both Can Tho and My Thuan stations represents the sediment flux by the Mekong River to the lower delta, we observed that the sediment supply varied strongly between 17.6 and 64.9 Mt yr−1 (Figure 3) with an average value of 37.5 Mt yr−1 (CV = 47%) during the study period. This average sediment supply is slightly lower to these estimated at Chau Doc and Tan Chau stations (about 100 km upstream from Can Tho and My Thuan) of 44.6 Mt yr−1 by the National Centre for Hydro-Meteorology [73] based on daily measurement of water discharge and sediment concentration during the 2008–2016 period. The specific sediment yield of the whole Mekong River watershed was estimated for the same period at 47 t km−2 yr−1. Water 2018, 10, x FOR PEER REVIEW 8 of 24

August 10,296 2.688 11,194 3.636 September 12,502 3.900 12,687 4.802 October 13,270 3.988 13,414 5.378 November 9785 1.696 9853 2.507 December 5624 0.882 6085 1.234

3.2. Annual Average and Interannual Variations The annual water discharge and sediment flux of the Mekong River at Can Tho and My Thuan stations calculated from the database were compiled in Table 4, and the values for the whole Mekong River, estimated as the summation of the two contributes, are presented in Figure 3.

Table 4. Annual water discharge and sediment flux at Can Tho and My Thuan stations during the 2009–2016 period.

Can Tho Station My Thuan Station Year Q Annual Qs Annual Q Annual Qs Annual (m3 s−1) (Mt yr−1) (m3 s−1) (Mt yr−1) 2009 6381 34.22 6948 30.70 2010 4929 11.13 5410 14.65 2011 7660 23.37 8572 38.32 2012 5774 12.48 6363 17.24 2013 6582 15.02 6888 23.70 2014 6482 16.39 6791 22.70 2015 4347 8.43 4737 9.15 2016 5151 8.51 5645 13.63 Min 4347 8.43 4737 9.15 Max 7660 34.22 8572 38.32 Water 2018, 10, 846 CV (%) 18.1 54.1 18.4 42.4 9 of 24 Average 5913 16.20 6419 21.26

80 20000 Annual mean water discharge (m3/s) Annual mean sediment flux (Mt/month) 16000

60 Water Flux (m

12000 40 8000 3 /s) 20 4000

Water 2018Sediment (Mt/yr) Flux , 10, x FOR PEER REVIEW 9 of 24

Assuming0 that the sum of sediment fluxes at both Can Tho and My Thuan stations0 represents the sediment flux2009 by the 2010 Mekong 2011 River to 2012 the lower 2013 delta, 2014 we observed 2015 that 2016 the sediment supply −1 −1 variedFigure strongly 3. Evolution between of annual17.6 and mean 64.9 water Mt yr discharge (Figure 3)(m with3 s−1) anand average sediment value flux of (Mt 37.5 yr Mt−1) of yr the (CV Mekong = 47%) Figure 3. Evolution of annual mean water discharge (m3 s−1) and sediment flux (Mt yr−1) of the duringRiver the from study 2009 period.to 2016. This average sediment supply is slightly lower to these estimated at Chau MekongDoc Riverand Tan from Chau 2009 stations to 2016. (about 100 km upstream from Can Tho and My Thuan) of 44.6 Mt yr−1 by theThese National results Centre show for Hydro-Methe high teorologytemporal [73]variations based on of daily annual measurement sediment of fluxes water indischarge the two and main 3.3. Trendsdistributariessediment in “In andconcentration of Out” the lower Fluxes during Mekong in thethe RiverEstuaries2008–2016 (Table peri 4).od. The The annual specific sediment sedime ntfluxes yield ranged of the fromwhole 8.43 Mekong River watershed was estimated for the same period at 47 t km−2 yr−1. (in 2015) to 34.22 Mt yr−1 (in 2009) around the average value of 16.20 Mt yr−1 at the Can Tho station, Thewith benefit a coefficient of regular of variation measurements of 54%. For the at My each Thuan flood station, and annual ebb sediment stages from fluxes 2009ranged to from 2016 is to 3.3. Trends in “In and Out” Fluxes in the Estuaries provide,9.15 for (in the 2015) first to time,38.32 Mt an yr estimate−1 (in 2011) of around both sediment the average fluxes value of flowing 21.26 Mt seaward yr−1, with anda coefficient landward of in the estuaries.variation TheThe monthlyof benefit 42%. of andregular yearly measurements values at are each presented flood and ebb in stages Figures from4 2009and to 20165. We is to observed provide, that the for the first time, an estimate of both sediment fluxes flowing seaward and landward in the estuaries. monthly evolutionThe monthly of and sediment yearly values flux flowingare presented seaward in Figures (flux-out) 4 and and5. We landward observed that (flux-in) the monthly at both stations experiencedevolution strong of seasonalsediment flux variations flowing seaward (Figure (flux-out)4). At theand Canlandward Tho (flux-in) station, at the both monthly stations sediment flux-out rangedexperienced between strong 0.14 seasonal and variations 10.5 Mt (Figure month 4).−1 At(average the Can Tho value station, of 1.5the Mtmonthly month sediment−1) with flux- the highest −1 −1 value observedout ranged inrainy between season. 0.14 and The 10.5 highest Mt month values (average of sediment value of 1.5 flux-in Mt month were) with observed the highest in dry season value observed in rainy season. The highest values of sediment flux-in were observed in dry season −1 −1 with individualwith individual values values varying varying from from 0.01 0.01 to to 0.45 0.45 MtMt month −1 (average(average value valueof 0.16 ofMt 0.16month Mt−1). month ). A similarA evolution similar evolution of sediment of sediment flux-in flux-in and and -out -out was observed observed at the at My the Thuan My Thuan station (Figure station 4). (Figure 4).

12 0.5 Can Tho Flux-out (Mt/month) 10 Flux-in (Mt/month) 0.4 8 0.3 6 0.2 4

2 0.1 Sediment flux-In (Mt/month) flux-In Sediment

Sediment flux-Out (Mt/month) flux-Out Sediment 0 0.0 Jul-16 Jul-11 Jan-14 Jan-09 Jun-14 Jun-09 Oct-12 Apr-15 Apr-10 Sep-15 Feb-16 Sep-10 Feb-11 Dec-16 Dec-11 Aug-13 Nov-14 Nov-09 Mar-13 May-12 14 0.3 Flux-out (Mt/month) 12 My Thuan Flux-in (Mt/month) 10 0.2 8 6 0.1 4 2 Sediment flux-In (Mt/month) flux-In Sediment Sediment flux-Out (Mt/month) flux-Out Sediment 0 0.0 May-… Jul-16 Jul-11 Jan-14 Jan-09 Jun-14 Jun-09 Oct-12 Apr-15 Apr-10 Sep-15 Feb-16 Sep-10 Feb-11 Dec-16 Dec-11 Aug-13 Nov-14 Nov-09 Mar-13 Figure 4. Evolution of monthly sediment fluxes oriented seaward and landward at Can Tho and My Figure 4. Evolution of monthly sediment fluxes oriented seaward and landward at Can Tho and My Thuan stations for the period 2009–2016. Thuan stations for the period 2009–2016.

Water 2018, 10, 846 10 of 24

In addition, we noted that the seaward flux was, in average 18 times the landward flux at My Water 2018, 10, x FOR PEER REVIEW 10 of 24 Thuan (22.52 Mt yr−1 vs. 1.26 Mt yr−1) and 9 times the landward flux at Can Tho (18.16 Mt yr−1 vs. −1 1.96 Mt yrIn addition,). The difference we noted that is likely the seaward due to flux the shortestwas, in average distance 18 betweentimes the landward Can Tho andflux at the My mouth thanThuan from My(22.52 Thuan Mt yr to−1 vs. its 1.26 mouth. Mt yr−1) and 9 times the landward flux at Can Tho (18.16 Mt yr−1 vs. 1.96 MtThe yr trend−1). The to difference a decrease is likely in the due yearly to the flux shorte flowingst distance out (Figurebetween5 Can) should Tho and be analyzedthe mouth withthan care, sincefrom the studyMy Thuan period to its encompassed mouth. an excess of discharge at the beginning of the study period with the La NiñaThe eventtrend ofto 2010–2011,a decrease in and the endedyearly flux with flowing a deficit out of (Figure discharge 5) should associated be analyzed to the with El Niño care, event of 2015–2016.since the study The fluxperiod flowing encompassed landward, an excess due of to di thescharge tidal at propagation, the beginning seemed of the study to be period not affected with by the La Niña event of 2010–2011, and ended with a deficit of discharge associated to the El Niño event the ENSO variation in Can Tho and slightly affected in My Thuan. The trend is almost nil at Can of 2015–2016. The flux flowing landward, due to the tidal propagation, seemed to be not affected by Tho,the while ENSO a short variation increase in Can in Tho landward and slightly sediment affected flux in My was Thuan. observed The trend at My is almost Thuan nil station at Can (FigureTho, 5). The differencewhile a short in theincrease trends in betweenlandward the sediment two stations flux was may observed be partly at explainedMy Thuan by station other (Figure factors 5). than El Niño,The such difference as sand in mining the trends activities between [66 the,67 two]. stations may be partly explained by other factors than ElIt mayNiño, be such useful as sand for mi modelingning activities purposes [66,67]. to provide the variations of average values of Ce and Cf at both stationsIt may be at theuseful monthly for modeling scale (Table purposes5). Theyto provide show the that variations SPM concentrations of average values are generallyof Ce and C higherf at Canat both Tho thanstations at Myat the Thuan monthly (both scale during (Table ebb 5). andThey flood show tides) that SPM during concentrations the dry season, are generally and that the higher at Can Tho than at My Thuan (both during ebb and flood tides) during the dry season, and reverse is true in wet season. The variations of Cf (CV = 35% at Can Tho, 51% at My Thuan) are smaller that the reverse is true in wet season. The variations of Cf (CV = 35% at Can Tho, 51% at My Thuan) than these of Ce (49% and 65%, respectively). The variations of Ce and Cf are very small during the low are smaller than these of Ce (49% and 65%, respectively). The variations of Ce and Cf are very small flow season, at Can Tho from November to June (CV = 5% for Cf and 24.5% for Ce) and at My Thuan during the low flow season, at Can Tho from November to June (CV = 5% for Cf and 24.5% for Ce) from January to May (CV = 6.7% for C and 16.6% for C ). During the year, the monthly variations are and at My Thuan from January to Mayf (CV = 6.7% fore Cf and 16.6% for Ce). During the year, the moremonthly amplified variations at My Thuanare more (CV amplified = 51% at for My Cf Thuanand 67% (CV for = 51% Ce) for than Cf atand Can 67% Tho for (CCVe) than = 35% at Canand 49%, respectively).Tho (CV = 35% and 49%, respectively).

45 5

) Can Tho Flux-Out Sediment flux - in (Mt yr (Mt in Sediment- flux -1 Flux-In y = -2.56x + 5165 4 30 R² = 0.51 3

2 15 y = -0.035x + 72.0 1 R² = 0.07 -1 ) Sediment Flux -Sediment out (Mt yr 0 0 2008 2010 2012 2014 2016

45 5 ) Sediment Flux - in (Mt yr in Sediment Flux -

-1 My Thuan Flux-Out y = -2.12x + 4282 4 R² = 0.30 Flux-In 30 3

2 15 1 y = 0.11x - 227 -1

R² = 0.62 ) Sediment Flux -Sediment out (Mt yr 0 0 2008 2010 2012 2014 2016

FigureFigure 5. Evolution 5. Evolution of of annual annual sediment sediment fluxesfluxes orient orienteded seaward seaward and and landward landward at Can at CanTho and Tho My and My ThuanThuan stations stations for for the the period period 2009–2016. 2009–2016.

Water 2018, 10, 846 11 of 24

Table 5. Average monthly concentrations at flood tide (Cf) and at ebb tide (Ce) at Can Tho and My Thuan stations during the 2009–2016 period.

Can Tho Station My Thuan Station Ratio Can Tho/My Thuan Month −1 −1 −1 −1 Cf (mg L )Ce (mg L )Cf (mg L )Ce (mg L ) Cf CT/Cf MT Ce CT/Ce MT January 31.18 38.17 23.95 32.26 1.30 1.18 February 32.07 38.44 24.40 32.42 1.32 1.19 March 34.51 36.70 22.49 27.16 1.53 1.35 April 35.11 35.36 22.90 23.87 1.53 1.48 May 30.84 31.81 20.50 22.56 1.50 1.41 June 32.59 46.61 33.29 47.75 0.97 0.98 July 58.05 74.81 56.34 90.11 1.03 0.83 August 68.19 92.61 72.35 114.85 0.94 0.81 September 68.01 117.92 91.35 142.03 0.74 0.83 October 49.90 106.82 60.74 138.58 0.82 0.77 November 33.63 63.55 46.81 92.46 0.72 0.69 December 31.03 52.82 41.19 70.12 0.75 0.75 CV (%) 35.13 49.09 51.31 64.85 28.92 27.86 Average 42.09 61.30 42.59 69.51 1.10 1.02

4. Discussion

4.1. Data Collection As before 2009, no regular accurate data on water discharge and sediment concentration was available on the lower Mekong River, the present results are important to document and improve our knowledge on the variability of hydrodynamics and suspended sediment transport in the Mekong estuary. However, two points may induce a bias or an uncertainty on sediment budget that we are not yet able to quantify: The use of filters of 8-µm porosity to retain the suspended material on the filter, and the use of only one value of suspended sediment concentration per tidal phase at each gauging station. Concerning the first point, filters of 8-µm porosity have been used since 2009 and the information on the kind of filter used before or upstream was not always documented. In the estuary, almost all suspended particles are present in the form of flocs with diameters bigger than 8 µm (see Gratiot et al. [79] on the Mekong River estuary, and Vinh et al. [80] on the Red River estuary—the neighbouring watershed). However, it remains possible that the use of filters of 8-µm porosity underestimates the total concentration of suspended matter; this may be the objective of a future analysis. More generally, we would like to warmly thank the Vietnamese authorities for their considerable effort in collecting these data and encourage them to give additional assessments on the values of sediment fluxes at each station in the future. Future data collection may also be inspired from feedbacks of long-term and continuous estuarine monitoring stations, which are still very scarce at global scale (e.g., [81–84]).

4.2. Factors Controlling Annual Sediment Transport in the Lower Mekong River Merging our data set and data from [85] (Figure 6) enabled us to check that the maximum and minimum water discharges over 17 years (2000–2016) were observed in 2011 and 2015. Although eight years of regular observation (2009–2016) in water discharge and suspended sediment concentration are not considered as “long-term survey”, however, the dataset collected during this period covered a large range of forcing and hydrological conditions. Water 2018, 10, 846 12 of 24 Water 2018, 10, x FOR PEER REVIEW 12 of 24

10000 MyThuan ) -1 s 8000 CanTho 3

6000

4000

2000 Water discharge (m discharge Water

0 2000 2002 2004 2006 2008 2010 2012 2014 2016

FigureFigure 6. 6.Variation Variation of of annual annual water water discharge discharge measured measured at at Can Can Tho Tho and and My My Thuan Thuan stations stations during during 2000–20162000–2016 period. period. 2000–2008: 2000–2008: Data Data from from [85 [85]] based based on on daily daily water water discharge; discharge; 2009–2016: 2009–2016: This This study. study.

4.2.1.4.2.1. Hydrological Hydrological Condition Condition TheThe temporal temporal variability variability of of annual annual sediment sediment fluxes fluxes at at both both hydrological hydrological stations stations during during the the 2009–20162009–2016 period period is is strongly strongly related related to to the the hydrological hydrological conditions. conditions. Extreme Extreme values values (highest (highest and and lowest)lowest) of of annual annual sediment sediment fluxes fluxes were were coincident coincident with with extreme extreme values values of of annual annual water water discharge discharge (except(except in in 2009 2009 at theat the Can Can Tho station,Tho station, Table Table4). To analyze4). To analyze more precisely more precisely the water-sediment the water-sediment relation, werelation, plotted we monthly/annual plotted monthly/annu sedimental fluxsediment against flux monthly/annual against monthl watery/annual discharge water discharge at Can Tho at and Can 2 MyTho Thuan and My during Thuan 2009–2016 during 2009–2016 period (Figure period7). (Figure The co-variation 7). The co-variation of Q and Qs of (withQ and R Qs2 ≥ (with0.62) R suggest ≥ 0.62) thatsuggest interannual that interannual hydrological hydrological variation plays variation an important plays an role important in sediment role supplyin sediment to the oceansupply by to the the Mekongocean by River. the Mekong River. TheThe year year 2009 2009 was was marked marked by by a a high high annual annual sediment sediment flux flux at at both both hydrological hydrological stations stations and and this this highhigh value value may may be be explained explained by by the the remobilization remobilization of of sediment sediment stored stored within within the the river river system system during during thethe previous previous years years of of low low water water discharge discharge (Figure (Figure6). 6).

4.2.2.4.2.2. General General Impact Impact of of ENSO ENSO on on Water Water and and Sediment Sediment Discharges Discharges InIn our our study study period, period, one one strong strong El El Niño Niño event even occurredt occurred in in 2015–2016 2015–2016 and and one one La La Niña Niña event event occurredoccurred in in 2010–2011. 2010–2011. The The analysis analysis of of correlations correlations between between ENSO ENSO indices indices and and water water and and sediment sediment suppliessupplies by by the the Mekong Mekong River River was was performed performed (Table (Table6 ).6). We We considered considered running running averaged averaged values values of of QQ and and Qs Qs over over 1, 1, 3 3 and and 5 5 months, months, and and running running averaged averaged SOI SOI values values over over 1 1 and and 3 3 months, months, then then we we calculatedcalculated the the correlation correlation coefficients coefficients between between series series (Q (Q vs. vs. SOI, SOI, Qs Qs vs. vs. SOI) SOI) with with different different time time lags. lags. CorrelationCorrelation coefficients coefficients were were systematically systematically higher higher with with SOI SOI averaged averaged over over three three months months than than over over oneone month. month. The The time time lags lags providing providing the the best best correlations correlations between between Q Q or or Qs Qs and and SOI SOI were were between between sevenseven and and nine nine months. months. Higher Higher lags lags (10 (10 months months or or more) more) gave gave smaller smaller correlations. correlations. TheThe results results obtained obtained suggested suggested that that if if the the interannual interannual signal signal of of water water discharge discharge in in the the lower lower MekongMekong River River have have been been regulated regulated by by other other factor factor (i.e., (i.e., dams), dams), water water flux flux of of the the Mekong Mekong River River seems seems toto respond respond to to ENSO ENSO with with a a 7–8 7–8 month month lag lag (r (r = = +0.337 +0.337 and and +0.343, +0.343, with with 7–8 7–8 month month lags, lags, respectively, respectively, TableTable6 ),6), and and sediment sediment discharge discharge with with a a 8–9 8–9 month month lag lag (r (r = = +0.511 +0.511 and and +0.521, +0.521, with with 8–9 8–9 month month lags, lags, respectively,respectively, Table Table6 ).6). This This result result is is in in agreement agreement with with Fok Fok et et al. al. [ 61[61]] who who found found a a time-lag time-lag of of six six monthsmonths between between water water levels levels and and SOI SOI during during the the El El Niño Niño event event of of 1991–1992 1991–1992 and and of of nine nine months months duringduring the the strong strong El El Niño Niño event event of of 1997–1998. 1997–1998.

Water 2018, 10, 846 13 of 24 Water 2018, 10, x FOR PEER REVIEW 13 of 24

14 12 Can Tho My Thuan 12 10 y = 10-6 x1.58 10 8 y = 2 10-7 x1.78 R² = 0.84 8 R² = 0.94 6 6 4 4 2 2 Sediment Flux (Mt/month) Flux Sediment 0 0 0 5000 10000 15000 20000 0 5000 10000 15000 20000 25000

40 45

2009 30 2009 30

y = 2 10-7 x2.08 -9 2.47 20 y = 8 10 x R² = 0.62 R² = 0.94 15 10 Sediment Flux (Mt/yr) Flux Sediment

0 0 0 2000 4000 6000 8000 10000 0 2000 4000 6000 8000 10000 Water discharge (m3/s) Water discharge (m3/s)

−1 FigureFigure 7.7.Relationships Relationshipsbetween between monthly monthly sediment sediment flux flux (Mt (Mt month month−1)) andand monthlymonthly waterwater dischargesdischarges 3 −1 −1 3 −1 (m(m3 s−1);); annual annual sediment sediment flux flux (Mt (Mt yr−1 yr) and) annual and annual water waterdischarges discharges (m3 s−1) (m at Cans Tho) at and Can My Tho Thuan and 2 Myfor the Thuan 2009–2016 for the period. 2009–2016 R2 values period. were R valuescalculated were including calculated all includingmonths (above all months) or all years (above (below) or all). years (below). Table 6. Correlation coefficients (r) at given lags (in month) between the running average of SOI over Table3 months 6. Correlation and the running coefficients average (r) atof givenwater lagsdischarge (in month) (Q) and between sediment the flux running (Qs) averageof the Mekong of SOI River over 3(sum months of fluxes and the at runningCan Thoaverage and My of Thuan) water over discharge 1, 3 and (Q) 5 and months. sediment flux (Qs) of the Mekong River (sum of fluxes at Can Tho and My Thuan) over 1, 3 and 5 months. Correlation Coefficient between SOI and Q or Qs Period of Q or Qs Averaging CorrelationQ Coefficient between SOI andQs Q or Qs 7 m lag 8 m lag 9 m lag 7 m lag 8 m lag 9 m lag Period of Q or Qs Averaging Q Qs 1 month 0.27 0.26 0.23 0.38 0.41 0.41 3 months 7 m lag 0.29 8 m lag0.28 9 m0.26 lag 70.41 m lag 0.45 8 m lag 0.45 9 m lag 1 month5 months 0.27 0.34 0.260.34 0.230.33 0.47 0.38 0.51 0.41 0.52 0.41 3 months 0.29 0.28 0.26 0.41 0.45 0.45 The correlation5 months is positive, indicating0.34 that water0.34 and sediment0.33 supplies 0.47 are reduced 0.51 in 0.52El Niño phases (SOI < 0) and enhanced in La Niña phases (SOI > 0). The co-variations of Qs averaged over 5 monthsThe and correlation SOI averaged is positive, over indicatingthree months that with water a andnine-month sediment lag, supplies is shown are in reduced Figure in 8. ElWe Niño can phasescheck easily (SOI < that 0) and the enhancedhighest fluxes in La in Niña 2009 phases and 2 (SOI011 are > 0). consistent The co-variations with positive of Qs SOI averaged values overnine 5months months before and SOI (in averagedparticular over between three La months Niña witheven at nine-monthof 2010–2011), lag, and isshown that the in smallest Figure 8 .fluxes We can of check2015 and easily 2016 that are the consistent highest with fluxes the in strong 2009 andEl Niño 2011 of are 2015–2016 consistent which with started positive to develop SOI values late 2014. nine monthsThe before resulting (in particular water discharge between Lais Niñanow eventmore ofregu 2010–2011),lated than and before, that theand smallest this regulation fluxes of 2015may andexplain 2016 the are weakness consistent of with the the correlation strong El between Niño of 2015–2016Q and SOI which at yearly started scale. to develop However late the 2014. erosion (andThe thus resulting the suspended water discharge sediment is nowconcentration) more regulated likely than remain before, varying and this with regulation the rainfall may explainregime theunder weakness the ENSO of the influence. correlation This between may explain Q and the SOI high at yearlyest correlation scale. However coefficients the erosionobtained (and between thus the Qs and SOI than between Q and SOI. The SPM concentration variation appeared to be more sensitive to the memory of ENSO than the water discharge, which is regulated by reservoir management.

Water 2018, 10, 846 14 of 24

suspended sediment concentration) likely remain varying with the rainfall regime under the ENSO influence. This may explain the highest correlation coefficients obtained between Qs and SOI than between Q and SOI. The SPM concentration variation appeared to be more sensitive to the memory of WaterENSOWater 2018 2018 than, 10, 10, x the, xFOR FOR water PEER PEER discharge,REVIEW REVIEW which is regulated by reservoir management. 1414 of of 24 24

1616 3 3 Qs 5-months running average (in Mt/month)

Qs 5-months running average (in Mt/month) SOI averaged over 3 months, 2.52.5 SOI averaged over 3 months, 1414 SOISOI 3-months 3-months running running average, average, with with 9 9months months lag lag 2 2 with 9 months lag 1212 with 9 months lag 1.51.5 10 10 1 1 8 8 0.50.5 0 6 6 0 (Mt/month) (Mt/month) -0.5-0.5 4 4 -1-1 2 Qs averaed over 5 months 2 Qs averaed over 5 months -1.5-1.5 0 0 -2-2 Jul-16 Jul-15 Jul-14 Jul-13 Jul-12 Jul-11 Jul-10 Jul-09 Jul-16 Jul-15 Jul-14 Jul-13 Jul-12 Jul-11 Jul-10 Jul-09 Nov-16 Nov-15 Nov-14 Nov-13 Nov-12 Nov-11 Nov-10 Nov-09 Mar-16 Mar-15 Mar-14 Mar-13 Mar-12 Mar-11 Mar-10 Mar-09 Nov-16 Nov-15 Nov-14 Nov-13 Nov-12 Nov-11 Nov-10 Nov-09 Mar-16 Mar-15 Mar-14 Mar-13 Mar-12 Mar-11 Mar-10 Mar-09

FigureFigureFigure 8. 8. Time Time variationvariation variation of of sedimentof sediment sediment flux flux flux of theof of the Mekong the Me Mekong Riverkong River (runningRiver (running (running averaged averaged averaged values values overvalues 5 months,over over 5 5 −1 months,expressedmonths, expressed expressed in Mt month in in Mt −Mt1 )month andmonth SOI−1) and (running) and SOI SOI (running averaged (running averaged value averaged over value 3value months), over over 3 with 3months), months), a time-lag with with ofa a 9time-lag monthstime-lag offorof 9 9 SOImonths months (i.e., for forfor SOI example:SOI (i.e., (i.e., for for the example: example: running the averagedthe running running value averaged averaged of SOI value value for January, of of SOI SOI for afor given January, January, year, a agiven is given shown year, year, here is is shownforshown October—9 here here for for October—9 months October—9 later—same mo monthsnths later—same year). later—same year). year).

4.2.3.4.2.3.4.2.3. Influences InfluencesInfluences of ofof ENSO ENSOENSO on on on Seasonal Seasonal Seasonal and and and Annual Annual Annual Sediment Sediment Sediment Supply Supply Supply InInIn order order order to to to assess assess assess the the the influence influence influence of of of ENSO ENSO ENSO on on on su suspended suspendedspended sediment sediment sediment fluxes fluxes fluxes at at at monthly monthly monthly and and and annual annual annual scales,scales,scales, we we we compared compared compared the the the monthly monthly monthly and and and yearly yearly yearly values values values of of of 2011 2011 2011 (affected (affected (affected by by by the the the 2010 2010 2010 La La La Niña), Niña), Niña), 2015–2016 2015–2016 2015–2016 (affected(affected(affected byby by thethe the strong strong strong 2015 2015 2015 El Niño), El El Niño), Niño), to the to onesto th th averagede eones ones averaged overaveraged 2009–2010 over over 2009–2010 and2009–2010 2012–2014. and and The2012–2014. 2012–2014. monthly ThevaluesThe monthly monthly are presented values values are inare Figurepresented presented9, and in in Figure theFigure yearly 9, 9, and and averaged the the yearly yearly values, averaged averaged per period, values, values, for per per Q period, and period, Qs, for arefor Q givenQ and and Qs,inQs, Table are are given given7. in in Table Table 7. 7.

2020 0.60.6 2015/2016 2015/2016 1818 2015/2016 2015/2016 NeutralNeutral Phase Phase 16 NeutralNeutral Phase Phase 0.50.5 16 2011 20112011 2011 1414 0.40.4 1212 1010 0.30.3 88 0.2 66 0.2 44 0.10.1 Sediment flux Out (Mt/month) Out flux Sediment

2 (Mt/month) In flux Sediment Sediment flux Out (Mt/month) Out flux Sediment

2 (Mt/month) In flux Sediment 00 0.00.0 JFMAMJJASONDJFMAMJJASOND JFMAMJJASONDJFMAMJJASOND

FigureFigureFigure 9. 9.9. MonthlyMonthly Monthly evolution evolutionevolution of ofof the thethe sedimentsediment sediment flux-out flux-outflux-out andand and flux-in flux-in flux-in of of of the the the Mekong Mekong Mekong River. River. River.

TableTable 7. 7. Average Average yearly yearly water water and and sediment sediment discharge discharges sat at Can Can Tho Tho and and My My Thuan Thuan stations stations during during differentdifferent ENSO ENSO stages stages over over the the 2009–2016 2009–2016 period. period.

CanCan Tho Tho My My Thuan Thuan Total Total Mekong Mekong PeriodPeriod FluxFlux FluxFlux FluxFlux FluxFlux FluxFlux FluxFlux TotalTotal TotalTotal TotalTotal InIn OutOut InIn OutOut InIn OutOut QQ 2011 2011 (La (La Niña) Niña) * * 47.9 47.9 290.2 290.2 242.3 242.3 36.7 36.7 307.9 307.9 271.2 271.2 84.6 84.6 598.1 598.1 513.5 513.5 QQ 2015–16 2015–16 (El (El Niño) Niño) * * 70.7 70.7 221.1 221.1 150.4 150.4 57.1 57.1 221.6 221.6 164.5 164.5 127.8 127.8 442.7 442.7 314.9 314.9 QQ neutral neutral phase phase * * 54.4 54.4 245.2 245.2 190.8 190.8 40.4 40.4 245.6 245.6 205.2 205.2 94.9 94.9 490.9 490.9 396.0 396.0 QsQs 2011 2011 (La (La Niña) Niña) ** ** 1.88 1.88 25.25 25.25 23.37 23.37 1.01 1.01 39.33 39.33 38.32 38.32 2.89 2.89 64.58 64.58 61.69 61.69 QsQs 2015–16 2015–16 (El (El Niño) Niño) ** ** 2.11 2.11 10.58 10.58 8.47 8.47 1.74 1.74 13.13 13.13 11.39 11.39 3.84 3.84 23.70 23.70 19.86 19.86 QsQs neutral neutral phase phase ** ** 1.93 1.93 19.77 19.77 17.85 17.85 1.11 1.11 22.91 22.91 21.80 21.80 3.04 3.04 42.68 42.68 39.65 39.65 * in* in km km3 yr3 yr−1;− 1**; ** in in Mt Mt yr yr−1.− 1.

Water 2018, 10, 846 15 of 24

Table 7. Average yearly water and sediment discharges at Can Tho and My Thuan stations during different ENSO stages over the 2009–2016 period.

Can Tho My Thuan Total Mekong Period Flux In Flux Out Total Flux In Flux Out Total Flux In Flux Out Total Q 2011 (La Niña) * 47.9 290.2 242.3 36.7 307.9 271.2 84.6 598.1 513.5 Q 2015–16 (El Niño) * 70.7 221.1 150.4 57.1 221.6 164.5 127.8 442.7 314.9 Q neutral phase * 54.4 245.2 190.8 40.4 245.6 205.2 94.9 490.9 396.0 Qs 2011 (La Niña) ** 1.88 25.25 23.37 1.01 39.33 38.32 2.89 64.58 61.69 Qs 2015–16 (El Niño) ** 2.11 10.58 8.47 1.74 13.13 11.39 3.84 23.70 19.86 Qs neutral phase ** 1.93 19.77 17.85 1.11 22.91 21.80 3.04 42.68 39.65 * in km3 yr−1; ** in Mt yr−1.

The influence of El Niño and La Niña on the monthly and yearly Q and Qs are very clear: In 2011 (affected by La Niña of 2010), the flux of water flowing to the sea increased by ~22% and the flux of water entering in the estuaries decreased by more than 10%. Globally, the net seaward flux of water increased by 29.6%. The sediment supply by the river increased by ~51% and the sediment flux inland decreased by 15%, providing a net increase of sediment supply to the sea by 55.6%. In 2015–2016 (affected by El Niño of 2015), the flux of water flowing to the sea decreased by ~10% and the flux of water entering in the estuaries increased by more than 34%. Globally, the net seaward flux of water decreased by 20.4%. The sediment supply by the river decreased by ~45% and the sediment flux inland increased by 26%, providing a net decrease of sediment supply to the sea by 50%. The flux back to the estuaries evolves the opposite of the flux seaward. The effect of ENSO on the sediment fluxes (in and out) is mainly sensitive in flood season (Figure 9). In conclusion, the water flux to the sea was 396 km3 yr−1 and the sediment supply to the sea was 39.65 Mt yr−1 in average over the neutral years. La Niña was seen to increase the water supply by almost 30% and the sediment supply by 55%. El Niño was seen to decrease the water supply by 20% and the sediment supply by 50%. The present water discharge of the Mekong River to the sea can be estimated to be 400 km3 yr−1, +/− 100 km3 yr−1 depending on ENSO, and the present sediment supply to the sea can be estimated to be 40 Mt yr−1, +/− 20 Mt yr −1 depending on ENSO. The supply of freshwater estimated by Milliman and Meade in 1983 (475 km3 yr −1)[4] was in the range of the present discharges, slightly overestimated as compared to the present discharge in neutral years. The loss of 20% is likely related to the increased water storage capacity (and thus increased evaporation), and to the irrigation needs. The average net water discharge was 12,550 m3 s−1 in neutral years. The maximum factor between monthly values was a factor of nine (between October during the flood period and the driest month of the February–April season) in neutral years, while it was 13 in La Niña period and seven during El Niño. These values may be compared to the values quoted in the literature of a 20-fold seasonal fluctuation from dry season (November–June) to wet (July–October) [86–88]. At least, the new dataset enabled to update the deliveries by each of the main distributaries. From the 2009–2016 measurements, the Tien River accounted for ~52% (51.8% in neutral years, 52.2% during El Niño and 52.8% during La Niña), and the Hau River for ~48% of the total Mekong River discharge, with small variations (of the order 1%) amongst the years. These values may be compared to former estimates: the Tien River accounted for nearly 58% of the total Mekong water discharge following Gugliotta et al. [50]. In terms of suspended sediment delivery to the sea, the Tien River accounted for a more variable portion of 55% in neutral years, 57.3% in 2015–2016 (El Niño) and 62% in 2011 (La Niña).

4.2.4. Looking for Breaking Points Many researches pointed out that the upstream Mekong developments such as the construction of dams has already led to significant trapping of sediment (up to 90% in the Manwan Dam, e.g., [36,47]). However, little information is available on impact of dams in the sediment flux in the lower Mekong Water 2018, 10, 846 16 of 24

River. The sediment flux loaded by the Mekong River to the ocean is still a big question, due to lack of long-term monitoring (before and after dam construction) and accurate database (e.g., daily/hourly measurement). During the study period, in the upper Mekong River basin, several big hydropower dams have been impounded (e.g., the Xiaowan Dam in 2010, the Jinghong Dam in 2011 and the Nuozhadu Dam in 2015; Table 2). In order to try identifying the impact of recent dams on sediment flux by the Mekong River to the estuary, the double-mass plot of annual sediment flux as a function of annual water volume for the Lower Mekong River during the 2009–2016 period was established (Figure 10). However, no inflection point was clearly observed. This result suggests that the sediment regime was not affected during the study period. However, the available data did not allow to analyze possible changes in the sediment regime before 2009. Using this methodology, breaking points were analyzed in the upper or middle Mekong River inWater different 2018, 10 studies, x FOR PEER (e.g., REVIEW [41,57 , 64]). Wang et al. [64] demonstrated that the sediment load at station16 of 24 Mukdahan in Laos did not decrease after the completion of the Manwan Reservoir (in 1993), but instead instead increased by ~50%. Liu et al. [57] based on annual sediment flux at different stations along increased by ~50%. Liu et al. [57] based on annual sediment flux at different stations along the Mekong the Mekong River (from Gajiu to Pakse stations) during the 1965–2003 period showed that the River (from Gajiu to Pakse stations) during the 1965–2003 period showed that the construction of major construction of major dams on the headwaters in China appears to have little impact on the sediment dams on the headwaters in China appears to have little impact on the sediment load at stations of the load at stations of the Low Mekong River by the early 2000s. They observed an inflection point around Low Mekong River by the early 2000s. They observed an inflection point around 1992–1993 at the 1992–1993 at the Gajiu station (just down the Manwan dam), right after its building. However, no or Gajiu station (just down the Manwan dam), right after its building. However, no or little impact was little impact was still noticed 400 km downstream in the early 2000s. still noticed 400 km downstream in the early 2000s. More recently, Nguyen et al. [41] based on monthly sediment concentration during the 1988– More recently, Nguyen et al. [41] based on monthly sediment concentration during the 1988–2010 2010 period measured at Chau Doc/Tan Chau and Can Tho/My Thuan stations concluded that after period measured at Chau Doc/Tan Chau and Can Tho/My Thuan stations concluded that after 2002 2002 (when the Dachaoshan reservoir completed), the sediment concentrations of the lower Mekong (when the Dachaoshan reservoir completed), the sediment concentrations of the lower Mekong River River slightly increased. The latter study assumed that this increase of sediment concentration may slightly increased. The latter study assumed that this increase of sediment concentration may be be explained by sand mining activity in the lower Mekong River basin. However, it is very important explained by sand mining activity in the lower Mekong River basin. However, it is very important to to underline that these results were based on low temporal resolution dataset (once per month). In underline that these results were based on low temporal resolution dataset (once per month). In order order to evaluate precisely sediment trapping of new reservoirs, a long-term observation (>10 years) to evaluate precisely sediment trapping of new reservoirs, a long-term observation (>10 years) is is needed. Moreover, despite sediment trapping is a logical consequence of water impoundments and needed. Moreover, despite sediment trapping is a logical consequence of water impoundments and a a long-term reduction in sediment availability downstream a dam is to be expected, the actual effects long-term reduction in sediment availability downstream a dam is to be expected, the actual effects on on the river system depend on a number of local and global factors taking place at different time the river system depend on a number of local and global factors taking place at different time scale, scale, so that it is difficult to derive general and immediate correlations [89]. so that it is difficult to derive general and immediate correlations [89].

400

300

200

100

Cumulated sediment supply (Mt) supply sediment Cumulated 0 0 1000 2000 3000 4000 Cumulated water supply (km3)

FigureFigure 10.10.Double-mass Double-mass plotsplots ofof annualannual sedimentsediment fluxflux andand annualannual waterwater fluxflux forfor thetheMekong Mekong RiverRiver (sum(sum of of fluxes fluxes at at Can Can Tho Tho and and My My Thuan Thuan stations) stations) during during 2009–2016 2009–2016 period. period.

TheThe onlyonly conclusionconclusion fromfrom thethe recentrecent databasedatabase isis thatthat wewe diddid notnotobserve observe anyany breakingbreaking pointspoints between 2009 and 2016. However, it is not informative of any variations before, especially in the between 2009 and 2016. However, it is not informative of any variations before, especially in the 2000s. Let us remember that Loisel et al. [58] showed that, from the full archives of MERIS data over 2000s. Let us remember that Loisel et al. [58] showed that, from the full archives of MERIS data the 2003–2012 period (~2000 satellite images), a very clear and regular decrease of suspended sediment concentration was observed along the Mekong delta coastal waters between 10 and 20 m depth. This decrease (of the order of −5% per year), accentuated in the wet season as compared to the dry season, was such that the difference between suspended sediment concentrations in dry and wet season kept going smaller so as to become almost equal in 2011. In another study of the shoreline change based on a 43-year Landsat image database (1973–2015), Li et al. [42] showed that the 600-km of coasts shifted from mainly accreted (at 68% between 1973 and 1988) to mainly eroded (at 56%, during 2005–2015). While most of the shorelines was in accretion until 2005, most has become eroded after 2005. Currently, 66% of the delta is under erosion, and the Mekong Delta has undergone a shift from growth to shrinkage since 2005 [42].

Water 2018, 10, 846 17 of 24 over the 2003–2012 period (~2000 satellite images), a very clear and regular decrease of suspended sediment concentration was observed along the Mekong delta coastal waters between 10 and 20 m depth. This decrease (of the order of −5% per year), accentuated in the wet season as compared to the dry season, was such that the difference between suspended sediment concentrations in dry and wet season kept going smaller so as to become almost equal in 2011. In another study of the shoreline change based on a 43-year Landsat image database (1973–2015), Li et al. [42] showed that the 600-km of coasts shifted from mainly accreted (at 68% between 1973 and 1988) to mainly eroded (at 56%, during 2005–2015). While most of the shorelines was in accretion until 2005, most has become eroded after 2005. Currently, 66% of the delta is under erosion, and the Mekong Delta has undergone a shift from growth to shrinkage since 2005 [42].

4.3. Spatial and Temporal Variation of Annual Sediment Flux in the Mekong River In order to analyze the spatio-temporal variation of sediment flux to the sea of the Mekong River over longer timescales, we merged our observational database and data extracted from previously published articles and reports: Annual data at Jiushou and Gajiu stations in the upper Mekong River during the 1965–2003 period were extracted from Fu and He [90], Fu et al. [47]; annual sediment loads in the middle-lower Mekong River (at Chiang Saen, Mukdahan and Pakse stations) came from the reports of the Mekong River Commission (during 1961–2002 and 2009–2013). In addition, we extracted annual sediment flux during the 2009–2016 period at Tan Chau and Chau Doc stations from the reports of the National Centre for Hydro-Meteorology in Vietnam. Moreover, the historical annual sediment flux load by the Mekong River to the ocean by Milliman and Syvitski [5] and Roberts [56] were used to compare with present sediment flux. Results are reported in Table 8 and Figure 11. Table 8 and Figure 11 show that the annual sediment flux load by the Mekong River has a strong and complex temporal/spatial variation. Qs increases from upstream to Pakse under the influences of tributaries, then decreasing values were measured from Pakse to the sea. From Pakse to the sea, a large portion of the sediment flux is deposited where the slope becomes mild [18], and/or due to water irrigation. At the Jiuzhou station, located 269 km upstream from the Manwan reservoir, the annual sediment flux during pre- and post-dam operation was relatively stable. In contrast, the sediment load at the Gajiu station, 2 km downstream from the reservoir, decreased to one-third of the pre-dam period. At the Chiang Sean station, the annual sediment flux has decreased from the 1960s on. The sediment load seemed to be not affected from Mukdahan to Pakse, during the 10 years after completion of the Manwan Reservoir. The slight increase of sediment flux at Pakse between 1986–2002 and 2009–2013 was likely related to land-use change and intensification of human disturbance in the lower Mekong basin during at last 20 years (e.g., [18]) and/or river bed-bank erosion due to imbalance between sediment transport and sediment trapped in the reservoir [41]. During the 2009–2016 period, the annual sediment flux transported from the middle to lower Mekong River has dramatically decreased in comparison with previous periods, while the annual water discharge during this period slightly increased. The sediment flux measured at Chiang Sean, Mudkahan and Pakse stations during the 2009–2013 period decreased by 65%, 35% and 62% as compared to 1993–2003. During 2002–2016, various big to medium dams were constructed in the main stream of upper Mekong River, including the Dachaoshan dam in 2002, the Xiawan Dam in 2012, the Jinghong Dam in 2013 and the Nuozhadu Dam in 2016 (Table 2). Although there is lack of continuous data (and in particular no data from 2003 to 2008), the marked reduction in sediment load in the Mekong River during 2009–2013 likely reflects sediment trapping. The available information derived from satellite data showed a decrease in surface SPM concentrations along the prodelta at a rate of ~5% per year during 10 years, in agreement with the decrease by 62% of Qs at Pakse between 1994–2003 and 2009–2016. Water 2018, 10, 846 18 of 24

Table 8. Summary of mean annual sediment flux (F in Mt yr−1) and annual average water discharge (Q in m3 s−1) in the Mekong River basin.

Before 1993 * 1994–2003 * 2009–2016 ** Q Qs Q Qs Q Qs (m3 s−1) (Mt yr−1) (m3 s−1) (Mt yr−1) (m3 s−1) (Mt yr−1) Jiuzhou (1965–2003) 25 27.5 Gajiu (1965–2003) 45.8 18.1 Chiang Saen (1961–2002, 2009–2013) 2917 71 2822 37 2502 12.8 Mukdahan (1961–2002, 2009–2013) 8974 144 6310 140 8530 91 Pakse (1986–2002, 2009–2013) 12,177 165 12,303 176 13,128 65.6 Tan Chau + Chau Doc (2009–2016) 13,302 45 Can Tho + My Thuan (2009–2016) 12,332 37.5 Historical delivery to the ocean 14,904 145–160

Water 2018, 10, x FOR* Monthly PEER REVIEW measurements; ** Daily measurements. During 2003–2008: no available data. 18 of 24

Figure 11. Mean annual sediment fluxes at stations along the Mekong River. Figure 11. Mean annual sediment fluxes at stations along the Mekong River.

Other causes of sediment reduction than reservoirs may be raised, related to the land use changesOther (LUC), causes in of particular sediment reductionto the reforestatio than reservoirsn and maysoil-conservation be raised, related in the to theupper land and use middle changes (LUC),Mekong in River particular basin. to Indeed, the reforestation the Chinese and Government soil-conservation launched in a the reforestation upper and and middle soil Mekongconservation River basin.program Indeed, in the theMekong Chinese River Government basin from 2000s, launched with athe reforestation objective of andwatershed soil conservation protection [57]. program The ineffect the of Mekong reforestation River programs basin from on 2000s, the sediment with the transport objective in ofthe watershed Mekong downstream protection [of57 the]. The existing effect ofmainstream reforestation dams programs is unknown on thebut sedimentmay have transport further reduced in the Mekongthe sediment downstream flux from ofbank the erosion existing mainstreamand landslides dams (e.g., is unknown [34,57]). butThus, may in have order further to deeply reduced analyze the sediment the impacts flux fromof the bank dams erosion on the and landslidessuspended (e.g., sediment [34,57 fluxes,]). Thus, the in impact order of to reforest deeplyation analyze in the the upper impacts basin of theneeds dams to be on assessed. the suspended sedimentFinally, fluxes, the thepresent impact sediment of reforestation supply by in thethe upperMekong basin River needs to the to beocean assessed. was estimated to be 40 MtFinally, yr−1, and the can present vary sedimentby +50% due supply to La by Niña the event Mekong or − River50% due to theto El ocean Niño wasevent. estimated The average to be −1 40value Mt yris a ,quarter and can of vary estimates by +50% (145–160 due to LaMt Niñayr−1) eventestablished or −50% before due the to El 1980s Niño [5,56,57] event. The (Table average 4). Although the sediment flux estimated by Milliman and Syvitski [5], Roberts [56] and Liu et al. [57] were based on limited dataset in terms of spatial coverage, temporal resolution and sampling frequency, the drastic reduction of sediment load to the ocean by the Mekong River in recent years likely results from important anthropogenic activities such as new dam construction and LUC.

5. Conclusions A new dataset is available on flow and sediment discharge in the lower Mekong from 2009 on. This is the first dataset based on regular measurements, at high temporal resolution (hourly for discharge, twice daily measurements for sediment concentration). Even if the representability of two sediment samples for the ebb and flow periods is questionable, these measurements are fully informative and bring fresh insights to the scientific community and to the river basin authorities. Based on high temporal resolution of water discharge and suspended sediment concentration at Can Tho and My Thuan stations during the 2009–2016 period, we observed that the annual sediment supplies by the Mekong strongly varied, in close dependence to the hydrological conditions.

Water 2018, 10, 846 19 of 24 value is a quarter of estimates (145–160 Mt yr−1) established before the 1980s [5,56,57] (Table 4). Although the sediment flux estimated by Milliman and Syvitski [5], Roberts [56] and Liu et al. [57] were based on limited dataset in terms of spatial coverage, temporal resolution and sampling frequency, the drastic reduction of sediment load to the ocean by the Mekong River in recent years likely results from important anthropogenic activities such as new dam construction and LUC.

5. Conclusions A new dataset is available on flow and sediment discharge in the lower Mekong from 2009 on. This is the first dataset based on regular measurements, at high temporal resolution (hourly for discharge, twice daily measurements for sediment concentration). Even if the representability of two sediment samples for the ebb and flow periods is questionable, these measurements are fully informative and bring fresh insights to the scientific community and to the river basin authorities. Based on high temporal resolution of water discharge and suspended sediment concentration at Can Tho and My Thuan stations during the 2009–2016 period, we observed that the annual sediment supplies by the Mekong strongly varied, in close dependence to the hydrological conditions. Considering that the total sediment flux of both (Can Tho and My Thuan) stations represents the suspended sediment supply by the Mekong River to the South China Sea, we estimated the actual annual suspended sediment supply to be now 40 Mt yr−1 in neutral years (+/− 20 Mt yr−1 following ENSO), corresponding to a specific sediment yield of 50 +/−25 t km−2 yr−1. This value is much lower than that estimated by Milliman and Syvitski [5] and Roberts [56] (~160 Mt yr−1) or by Liu et al. [57] (~145 Mt yr−1) during previous periods, before 2003. The strong influence of ENSO can be underline. It is more important on the sediment concentration C or flux Qs than on the water discharge Q, likely because of the water regulation along the Mekong River. Merging our database with those of the literature, we observed that sediment from upstream of the Mekong River are largely trapped within the dam/reservoirs (e.g., Manwan and Dachaoshan) from decades, and that the lower Mekong was not significantly affected before 2002–2003. Sediment load in the lower Mekong basin and sediment supply to the sea started to be significantly affected during the 2000s by various anthropogenic activities such as dams, sand mining, reforestation and soil-conservation, stated by a huge decrease in sediment load at Pakse (−62% between 1994–2003 and 2009–2016) and a decrease by 5% per year of the surface suspended sediment concentration in the coastal waters between 10 m and 20 m depth, from 2003 to 2012 [58]. The measured net suspended sediment load over the period 2009–2016 as compared to previous estimates of sediment supply in the lower Mekong delta are consistent with such a scenario of a huge decrease in the 2000s, with a reduction from 145–160 Mt yr−1 to ~40 Mt yr−1 (by ~70–75%) in three or four decades. These estimates are consistent as well with the observed increase of shoreline erosion (with currently 66% of shorelines eroded) and the decrease of Mekong Delta area, since 2005 [42]. Tidal pumping still continues to bring sediments from the coast upstream in the estuary, especially in the dry season (Figure 4, see also [72]). The landward sediment flux likely increases when the water discharge decreases due to irrigation, evaporation in the reservoirs, water regulation or El Niño events. The net sediment supply from the watershed to the sea was seen to decrease recently by ~75%. The alongshore sediment transport mainly depends on wave action [91,92]. Waves from the South and Southwest induce coastal erosion during the low flow season and govern a net transport towards the Northeast of the delta, at the annual scale [91]. The coastal sediment budget seems thus oriented towards a global accelerated coastal erosion in the following years and decades, especially along the southwestern part of the delta [43], and the sediment supply may continue to decrease in a foreseeable future, like suggested by Liu et al. [57], Kondolf et al. [88]. If nothing will be done to limit the sediment storage along the reservoirs and the water pumping in the delta, the delta might disappear by the end of the century [93]. Protection or restoration measures should then be encouraged in a source-to-sink perspective (the watershed-estuary-coastal zone continuum), which requires to rapidly set up monitoring, understanding and management tools adapted at the full basin scale. Water 2018, 10, 846 20 of 24

Author Contributions: S.O. and D.T.H. conceived and designed the study; G.V.V., D.T.H. and S.O. analyzed the data; D.T.H. and S.O. wrote the paper. Funding: This research was funded by the Research Project “The erosion process in the Lower Mekong Delta Coastal Zones (LMDCZ) and the measures for protecting Go-Cong and U-Minh from coastal erosion”, supported by the European Union (EU), the Agence Française de Développement (AFD) and the Southern Institute of Water Resources Research (SIWRR). Conflicts of Interest: The authors declare no conflict of interest.

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