How Tidal Processes Impact the Transfer of Sediment from Source to Sink: Mekong River Collaborative Studies

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CITATION Ogston, A.S., M.A. Allison, R.L. McLachlan, D.J. Nowacki, and J.D. Stephens. 2017. How tidal processes impact the transfer of sediment from source to sink: Mekong River collaborative studies. Oceanography 30(3):22–33, https://doi.org/10.5670/ oceanog.2017.311.

DOI https://doi.org/10.5670/oceanog.2017.311

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DOWNLOADED FROM HTTP://TOS.ORG/OCEANOGRAPHY SPECIAL ISSUE ON SEDIMENTARY PROCESSES BUILDING A TROPICAL DELTA YESTERDAY, TODAY, AND TOMORROW: THE MEKONG SYSTEM

How Tidal Processes Impact the Transfer of Sediment from Source to Sink MEKONG RIVER COLLABORATIVE STUDIES

By Andrea S. Ogston, Mead A. Allison, Robin L. McLachlan, Daniel J. Nowacki, and J. Drew Stephens

22 Oceanography | Vol.30, No.3 ABSTRACT. Significant sediment transformation and trapping occur along the tidal characterized by a network of tidal dis- and estuarine reaches of large rivers, complicating sediment source signals transmitted tributary channels surrounded by low- to the coastal ocean. The collaborative Mekong Tropical Delta Study explored the lying vegetated floodplains where agri- tidally influenced portion of the Mekong River to investigate processes that impact cultural practices control inundation. The mud- and sand-sized sediment transport and deposition associated with varying fluvial Mekong River can be divided into five and marine influences. Researchers participating in this 2014–2015 project found that sections. The freshwater river (fluvial end as sand and mud progress down the tidal portion of the river, sands in suspension member) extends from the headwaters can settle during reduced or slack flows as river discharge becomes progressively more to >300 km from the ocean and is not affected by tides in the seaward direction. Consequently, deposits on the tidal river affected by tides or other ocean processes. bed are connected to sand transport in the channel. In contrast, fine mud particles The tidal river begins ~300 km from the remain in suspension until they reach an interface zone where waters are still fresh, but ocean where the flows become affected by the downstream saline estuary nonetheless impacts the flows. In this interface zone, tidal processes that increase in intensity as within the estuary, fine particles tend to settle, draping the sand beds with mud and approaching the river mouth. Next comes limiting the connection between the bed and suspended sand. In the Mekong system, an intermediate section called the inter- the interface and estuarine zones migrate along the distributary channels seasonally, face, which occurs within the tidal river resulting in variable trapping dynamics and channel bed texture. Therefore, the but upstream of where the river converges signature of fluvial-sediment discharge is altered on its path to the coastal ocean, and with saline ocean waters; it is influenced the disconnected mud and sand supply functions at the river mouth should result in by the downstream estuary where fresh- distinct offshore depositional signatures. and saltwater mix. At most, the estuary can penetrate ~40–50 km into the distrib- INTRODUCTION Not traditionally belonging to the utaries. Finally, brackish and/or riverine Rivers are the largest suppliers of particu- research realm of river geomorphologists waters discharge into the ocean, referred late material to the world ocean, and they or of estuarine/coastal oceanographers, to as the marine end member of the sys- are estimated to deliver ~13 Gt yr–1 today the tidal river has generally been an tem. Thus, between the fluvial and coastal- (~15–17 Gt yr–1 naturally prior to major understudied part of the source-to-sink marine end members lie the tidal river, dam installation; Syvitski and Kettner, sedimentary system. A significant amount the interface, and the estuary over which 2011). Of this amount, the world’s larg- of sediment may be trapped within this sedimentary signals are altered. est rivers discharge a disproportionate gateway region. In large, tropical systems, For any individual river, the sediment volume of sediment to the deltas of the tidal rivers can stretch over hundreds of load reaching the ocean is typically esti- world. Subaerial deltaic regions associ- kilometers due to the broad, relatively mated from information obtained at the ated with large rivers tend to form near low-lying coastal plains they transit. Some lowermost river gauging station, which is their land-sea boundaries from sediment studies suggest that as much as one-third almost always upstream of tidal influence. delivered by the rivers. Deposition results of the sediment load carried seaward by For the major rivers of the world, the tidal from river deceleration associated with these rivers may be trapped within the river can extend inland for hundreds of flatter terrain and declining water surface tidal rivers (e.g., Ganges-Brahmaputra: kilometers. Thus, the hydrodynamic and slopes. Vast human populations inhabit Goodbred and Kuehl, 1999; Amazon: sedimentary processes in these long tidal these low-lying plains, harvesting rich Nittrouer et al., 1995) through accumula- sections of river remain poorly under- native fisheries and utilizing large areas tion in tidal floodplains, or even by chan- stood. As particles transit downstream for aquaculture and agriculture in the nel aggradation. Given the tremendous along a deltaic distributary channel, they fertile river-delivered soils. Subtle climate sediment transformation and trapping are subject to changing water flows: from variability and resulting sea level rise, as that may occur in tidal rivers, studies that steady unidirectional, to tidally unsteady, well as land use alterations and damming examine the environmental components, to tidally reversing, and ultimately to in the drainage basin, alter the ability hydrodynamic processes, and sediment estuarine mixing. Along the majority of of these deltaic regions to support ever- dynamics are critically needed within this continuum, knowledge is lacking expanding populations. In order to man- this region to better understand the full about how the pathways and processes of age and maintain the subaerial deltaic source-to-sink system. mud and sand transport evolve. plains that result from the interplay of The tidal Mekong River (Figure 1) The collaborative 2014–2015 Mekong riverine (or fluvial) and marine processes, is an excellent place to study mud- and Tropical Delta Study (and prelimi- it is increasingly important to under- sand-sized sediment dynamics in a tidal nary studies in 2012–2013) provided an stand the dynamics that supply fresh- river and estuary and to explore the unprecedented opportunity to focus on water and sediment to and through these evolution of channel morphology. The sediment dispersal in a tidal river and expansive lowland reaches. large tide-dominated Mekong Delta is its intertidal flanks, shoreline deposits

Oceanography | September 2017 23 5 x 104 )

–1 4 s

3 Kratie 3

Discharge (m Discharge 1 Can Tho 0 Jan Mar May Jul Sep Nov Jan

FIGURE 1. Lower tidal portion of the Mekong River, site of distributary channel investigations that were part of the 2014–2015 Mekong Tropical Delta Study. The river starts to bifurcate downstream of Kratie, Cambodia, breaking into seven distributaries. The two largest are the Song Hau and the Song Tien. This article focuses on the Song Hau, which is reported to carry ~40% of the total Mekong water discharge (Nguyen et al., 2008). Note that tides propagate up ~300 km upstream of the river’s mouth, but saline marine waters only reach ~40 km upstream. Discharge data shown are the historical average (± one standard deviation; dashed lines) based on 45 years of data at Kratie and seven years at Can Tho (inset after Nowacki et al., 2015).

colonized by mangroves, and linkages tidal river influences should be detectable the mouth (Milliman and Meade, 1983). with shallow deposits on the continental in the signatures of sediment flux and The lower river basin and Mekong shelf. Additionally, these studies permit- deposition found there. Delta are located in a wet tropical set- ted an assessment of the role that climate In this paper, we first summarize the ting (~8.5°N–10.5°N), where the system and human impacts, such as sea level rise characteristics at the ends of the system— receives ~80% of its precipitation in May and damming of the upper river, play on the fluvial and marine components. We through September during the southwest sediment dynamics. then discuss the processes within the tidal monsoons (Mekong River Commission, Our goals have been to identify and river and estuary that form the pathways 2005). At the Kratie gauging station, quantify the range of dynamical pro- and sinks for sediment and that deter- 500 km upstream of the mouth, river dis- cesses along the river-to-ocean progres- mine the bed morphology of the distrib- charge reaches its maximum in August– sion described above. Understanding utary channels. Finally, we discuss possi- September and its minimum in March– these processes allows a better concep- ble Mekong distributary evolution based April (Figure 1). This seasonal water tualization of the formation and evolu- on these processes. discharge is modified by Tonle Sap lake, tion of deltaic distributary channels, and which acts as a natural flow capaci- of prograding/eroding channel islands in TWO ENDS OF THE MEKONG tor, smoothing out seasonal variabil- reaches where there are multiple chan- RIVER SYSTEM ity at locations downstream (see delay nel courses. The controls within the tidal The Fluvial End Member in discharge peak between Can Tho and river alter the magnitude and timing of The Mekong River originates on the Kratie, Cambodia, in Figure 1). At high the fluvial-sediment supply signal before Tibetan Plateau of China, where it flow, freshwater extends to the mouths discharge to the continental shelf. Where derives much of its sediment load of of the distributary channels, but during deltaic deposits are formed either at the ~160 × 106 t yr–1, as estimated at Phnom low flow, saltwater penetrates ~40–50 km shoreline or in subaqueous environments, Penh, Cambodia, 300 km upstream of up river (Wolanski et al., 1996, 1998;

24 Oceanography | Vol.30, No.3 Nowacki et al., 2015). Commission, 2005). Offshore, sedimen- sand-sized sediment along the region and Suspended sediment loads at Kratie, tary processes are further affected by sea- channel morphologies resulting from the gauging station located upstream sonal wind and wave activity and by circu- these processes. of the delta, have been recently re- lation driven by buoyant discharge at the evaluated and are estimated to fall in the coast (e.g., Eidam et al., in press). Seaward Pathways and Sinks of range of 80–110 Mt yr–1 (Milliman and of the mouth of the largest Mekong dis- Fine-Grained Sediment Farnsworth, 2011), with recent data sug- tributary, the Song Hau (also known as We focused our investigations on the gesting that the lower limit may be more the Bassac River), seasonal monsoon sys- Song Hau distributary of the Mekong appropriate (Darby et al., 2016). The sed- tems dictate wave and current properties. River, where prior studies suggested that iment load at Kratie is highly seasonal, The rainy (southwest) monsoon (May– salinity intruded into the lower ~50 km with little appreciable load from January September) generally exhibits lighter of the distributary during seasonal low- to June. These estimates are made above winds, resulting in slow currents toward river flows, and was confined near the the bifurcation of the river into distrib- the northeast. The windy and dry (north- mouth during high river flows (Wolanski utary channels and at a location hun- east) monsoon (October–April) is charac- et al., 1996, 1998). During high-flow con- dreds of kilometers upstream of the river terized by strong alongshore trade winds, ditions, Wolanski et al. (1996) observed a mouth that has little to no influence from persistent wave activity with associated salt-wedge estuary and sediment export tidal processes and therefore may have sediment resuspension, and coastal cur- to the ocean. Ebb flows (tide plus river little connection to the sediment dis- rents toward the southwest. flow) were stronger than flood flows charge patterns and fluxes at the entrance Not only is the marine end member (tides minus river flow), the river bed was to the coastal ocean. the recipient of the fluvial discharge sig- mostly sandy, and no along-channel gra- There has been considerable concern nal, there is also the potential for sedi- dient in suspended sediment concentra- about the influence of anthropogenic ment previously debouched to the shelf tion was observed. A near-bed maximum activities on the sediment load deliv- to be imported into distributary channels in turbidity was present, known as the ered to the delta, specifically focused during reduced river flow (Nowacki et al., estuarine turbidity maximum, which is on sediment limitation resulting from 2015). Accordingly, offshore processes seen in many estuarine systems. During upstream damming and sand min- need to be considered for a full under- low-flow conditions, Wolanski et al. ing in the deltaic distributary channels. standing of the sediment source func- (1998) found the Mekong estuary to be Furthermore, Darby et al. (2016) report tion at the mouth. In large river systems, partially mixed, with stronger peak flood that changes to climate patterns, includ- understanding the linkages between the flows than ebb flows. Suspended sedi- ing a changing tropical cyclone climatol- tidal river, the estuary, and offshore envi- ment, potentially deposited on the inner ogy, have drastically reduced the amount ronments is necessary to conceptualize shelf during a previous high-flow period, of suspended sediment leaving the fluvial the deltaic processes on time scales of may have been transported back from system. In all, the absolute amount of hours to millennia. coastal waters into the river mouth and sediment leaving the purely fluvial end trapped by estuarine processes. member as suspended load is not well PROCESSES WITHIN THE TIDAL The seasonal experiments conducted established, yet recent studies suggest RIVER AND ESTUARY in the Mekong tidal river as part of our that it has been reduced on the order of There is significant potential for sediment collaborative international program in 25% over the past 25 years (Kummu et al., entrapment in the tidal river and estuary, 2014–2015, including preliminary stud- 2010; Kondolf et al., 2014). which would affect the amount and tim- ies in 2012–2013, were designed to fur- ing of sediment discharge to the ocean ther explore the range of transport and The Marine End Member as well as permit deposition along the deposition processes not only in the At the marine end of the tidal river, oce- channel bed. In these zones, mud- and estuary but also in the upstream tidal anic processes drive fluctuations in river sand-grained sediments appear to fol- river reach. We emphasized a dynami- discharge and carrying capacity that low different pathways, as the deceler- cal understanding of transport processes, impact the sediment delivery through ation and acceleration of the tidal flows including evaluation of bed stresses (the this reach. The coastal water levels at segregate particles by settling veloc- stress induced on the channel bed sedi- the mouth of the Mekong River exhibit ity. Between the fluvial- and marine- ment by the flows above it) and resuspen- mesotidal fluctuations, with semidiurnal dominated end-member states, sediment sion of distinct sand and mud popula- ranges during spring tides of ~3.5 m at the dynamics vary due to fluctuating magni- tions, and we focused on the connections ocean boundary, decreasing to ~2.0 m at tudes of fluvial and marine influences in between the transport dynamics and the 90 km upstream (Can Tho) and to ~1.0 m the tidal river. In this section, we discuss channel bed. These studies within the at 190 km upstream (Mekong River the processes affecting both mud- and tidal Mekong River focused on the lower

Oceanography | September 2017 25 80 km of the Song Hau from the city of turbidity maximum, influence movement a short phase of the tide. As a result, fine- Can Tho to the mouth, where flows are of salt and fine-grained sediment through grained sediment is exported when river discharged into the East Sea (also known the system. This results from variations in discharge is high. Residual sediment flux as the South China Sea). The Song Hau is bed stress, particle aggregation (floccula- is seaward throughout the water column the largest distributary channel and car- tion), settling, and trapping of sediment during high flow. Decomposition of sed- ries ~40% of the water discharge of the within the distributary channel. iment flux terms shows that during high Mekong (Nguyen et al., 2008). Within the The flow and sediment dynamics of the flow, over 80% of the total sediment flux Song Hau distributary, water discharge tidal Mekong River undergo a compre- could be attributed to advection by the fluctuates (and reverses at the seaward hensive seasonal regime change between river, with most of the remainder con- end) with the tide: the discharge vari- high and low river discharge. This change tained in the tidal term, which encom- ation is seasonally most asymmetric at is highlighted at the downstream limit passes processes such as tidal pumping high river discharge with flood and ebb by the seasonal transition from a tidal and local resuspension and deposition. discharge rates and duration out of bal- freshwater system with an ephemeral In contrast, during low flow, the sea- ance over the tidal cycle (Figure 2), and salt wedge to a partially mixed estuarine ward portions of distributaries become spatially becomes increasingly asym- system (Figure 4; Nowacki et al., 2015). partially mixed estuaries, the net flux is metric with distance upstream. Overall, During high river discharge, estuarine inland, and sediment is imported from we found that tidal discharge fluctua- mixing generally occurs outside the river the offshore regime. The flux decom- tions (Figure 3) in the river, along with mouth, and the salt wedge penetrates the position demonstrates that the trans- the periodic presence of an estuarine distributary channel mouths only during port is primarily associated with estuarine exchange and tidal processes

Distance Upstream (km) (e.g., tidal pumping, resuspension) and a 200 160 120 80 40 0 the river term is generally found to be 0 unimportant. Baroclinic and tidal pro- TIDAL 10 unidirectional cesses, especially local resuspension, are reversing 20 c more important during low flow. The t C t B t A o ESTUARINE

Th resulting residual sediment flux is land- 30 er Mouth salt wedge ansec ansec ansec ward in deep regions and seaward in the Can Chau Do Riv Tr Tr Tr partially mixed 40 upper water column.

Depth Below MSL (m) High Flow 50 Low Flow The freshwater tidal zone upstream of the estuary is of particular interest b because processes there, such as bed shear

1 Can Tho stress, are impacted by downstream strat- Trans C 0 Dinh An ified flow dynamics in the area we refer −1 to as the interface zone (McLachlan et al., in press; Figure 4b). Convergence of fine-

Water Level (m) 00:00 06:00 12:00 18:00 00:00 Time in Tidal Cycle (hh:mm) grained sediment is enhanced due to the c 4 x 10 slower flows and reduced bed stresses in 4 High ow A B Low ow A B C m this region as well as in the estuary, pro-

) moting settling and deposition. In addi- –1 2 tion, particles that had aggregated in the ownstrea 0 salt front when the estuary was located at mD its landward maximum within the tidal

Discharge (m s −2 cycle continue to be available for resus-

Upstrea pension in freshwater when the estu- −4 ary retreats on ebb flows. Thus, although 06:00 18:00 06:00 06:00 18:00 06:00 estuarine processes are understood to be Time in Tidal Cycle (hh:mm) important drivers of flow convergence FIGURE 2. (a) Along-channel characteristics of the Song Hau distributary, including depth and tidal/ estuarine influences. The distributary channel is considered a tidal river upstream of the mouth (Nowacki et al., 2015), the Mekong stud- to locations above Chau Doc. (b) Water surface variations relative to the mean water level under ies also identified the interface region as high discharge conditions, illustrating the attenuation of the tidal range and progressive asymme- an important processing area for sed- try. After McLachlan et al. (in press) (c) Mekong River discharge under high and low river discharge conditions at transects shown in Figure 1. Note that discharge at Transect C is larger than at A and iment prior to introduction into the B due to the bifurcation in the distributary channel. After Nowacki et al. (2015) region classically considered estuarine.

26 Oceanography | Vol.30, No.3 In addition, changes in fine-grained river mouth influences the spatial distri- to considerable mud deposition. Bottom particle characteristics and settling veloc- bution and transport pathways of sedi- mapping and sampling of the reversing ity affect the ability of tidal and fluvial ment and salt, with flow in the Tran De tidal and estuarine portion of the Song processes to maintain sediment in sus- subchannel exhibiting more conver- Hau distributary channel in high- and pension through the interface and estu- gent flow, priming conditions for par- low-discharge conditions confirm this arine reaches. In this large tropical river, ticle deposition. Applying some sim- (Nowacki et al., 2015; Allison et al., in some of the fine particles transit the tid- ple assumptions about yearly river flow press), and allow us to further examine ally influenced reach as flocs, or aggre- patterns yields an annual Dinh An sub- these impacts. gated particles. Flocculation occurs channel suspended sediment discharge within the freshwater of the tidal river of ~9 Mt yr–1. Upscaling this value to The Pathways and Sources of upstream of the saline limit in the dis- estimate the overall rate for the Mekong Sand-Sized Sediment tributary, and flocs are particularly evi- River, we suggest that sediment discharge Although on a global basis the majority dent during low-flow conditions. Camera at the distributary mouths is ~40 Mt yr–1 of sediment export to the coastal marine analysis of sediment in suspension shows (Nowacki et al., 2015), a value lower than environment is made up of mud-sized the fine-grained fraction has a floccu- that found in earlier studies by Harden particles (<63 µm), sand (63–2,000 µm) lated component during low discharge and Sundborg (1992) and Milliman and is an important component of the struc- at Can Tho where no salt is ever seen but Farnsworth (2011) but comparable to the ture and maintenance of deltas. Thus, its where reversing tidal flows are observed. recent Darby et al. (2016) estimate. delivery in times of sea level change and Flocs in the freshwater reach of the tidal Seasonal flow and sediment trans- human-caused supply interruption is crit- river can have diameters of >200 microns port dynamics through the distribu- ical to maintaining the health of the delta (McLachlan et al., in press), and aggrega- tary channels are reflected in channel and of those who live upon it. Sand may tion is likely enabled by natural organic bed grain size characteristics. As a result, transit the tidal river continuum either coatings (Figure 5). The amount and size solely based on water column dynamics, as bedload or suspended load according of flocculated particles in the surface we can assume that bed grain size would to the strength of the ebb and flood cur- water remain relatively uniform down- be coarser during high flow and finer rents within the tidal river and estuary. stream through the Dinh An subchannel during low flow, and that imported mud In the Mekong River, sand-sized material of the Song Hau distributary channel, to the Tran De subchannel would lead was found to be a significant component where salt is present in high concentra- tions at depth but is not typically well High Flow: Low Flow: mixed throughout the water column. The Tidal River Tidal River to Estuary aggregated fraction increases by ~25% in the Tran De subchannel relative to that in the Dinh An subchannel during low flow because vertically mixed salinity supports inter-particle bonding. Ebb-to-Flood Ebb-to-Flood The regime change that takes place in the tidal Mekong River has import-

ant implications for suspended sediment Eb b import and export to/from the estuary and interface region. During high flow, Flood Flood the Song Hau is estimated to export sed- Flood iment to the shelf at a rate of ~1.7 t s–1 (Nowacki et al., 2015), with the Dinh An subchannel, the largest of the two subchannels, being responsible for about ~60% of the total sediment flux (Nowacki Flood-to-Ebb Flood-to-Ebb et al., 2015; McLachlan et al., in press). FIGURE 3. Sections of water velocity across Transect B in Figure 1 at three tidal phases (red regions During low flow, the Song Hau imports represent downstream flow and blue regions upstream flow). Under high-flow conditions, when sediment from offshore at about 0.25 t s–1, Transect B is upstream of estuarine processes, the flows are not solely influenced by the topography with most of it traveling through the at the specific cross section but are also influenced by the downstream saline wedge and channel morphology near the mouth. Under low-flow conditions, when Transect B is within the estuary, lat- smaller Tran De subchannel. The pres- eral variations in flow across the transect suggest local control with flood flows initiating near bed ence of a large mid-channel island at the across the transect, and ebb flows initiating at the surface over the shallows.

Oceanography | September 2017 27 of the total suspended sediment export down the tidal river and through the estu- stresses peak at similar values as those to the coastal ocean, and may repre- ary. During the high-discharge season, in the high-discharge season because the sent 5%–20% of the modern estimates suspended sand transport is controlled tidally varying discharge values dominate of suspended sediment export (Stephens by complex relationships between bed the discharge signal, and yet suspended et al., in press). Sand transport processes material, tidal flow, distance upstream, sand transport through the seaward seg- change dramatically between the high- and water discharge. With their rapid set- ment of the tidal river is almost com- and low-discharge seasons in this sys- tling velocity, sand particles tend to fall pletely shut down. Correlation between tem, and in a different manner than mud out of suspension during the brief but sand fluxes and bed material suggests that transport processes change. This differ- regularly occurring slow/slack tidal flows this shut down is due to mud mantling of ence creates a segregation of mud and when bed shear stress is at a minimum. the channel floor, which restricts access sand pathways as both fractions progress During the low-discharge season, bed to the local sand source in the channel beds (Allison et al., in press; McLachlan et al., in press; Stephens et al., in press). As the Tran De subchannel has a gen- Tidal River Interface Estuary erally muddier bottom in all seasons, Flood the larger Dinh An subchannel domi- Salt A C B Limit nates sand transport. D Techniques typically used in fluvial B' environments to evaluate size-dependent Mud Export sediment fluxes are challenging to use A' under tidally variable discharge conditions (see Box 1). However, Stephens et al. (in press) show that sand flux is gen- 10 km Ebb Salt Limit erally related to the tidally varying dis- 10 km N HIGH FLOW a charge of the river (Figure 6). They use modeled river discharge conditions to Tidal River Interface Estuary estimate that a net value of 6.5 Mt yr–1

Ebb of sand may be exported from the sum Salt B Mud Deposit A of the Mekong’s distributaries based on C Limit D present conditions in the Mekong tidal B'

Mud Mud 0.4 Export Convergence A' )

–1 Surface s Near Bed Mud 0.3 Import –3 elocity (cm

LOW FLOW V b 0.2 ρ = 2.65 g cm 01020 Tidal River Interface Estuary 0.1 –3 icle Settling

Export Convergence rt ρ = 1.15 g cm Pa 0.0 Import 0 20 40 60 80 100 Particle Diameter (μm)

Bed Stress: Mud Deposit FIGURE 5. Particle size and settling velocity c High Low Low as obtained from a settling chamber. Although particles are likely somewhat disturbed prior FIGURE 4. Conceptual diagram of sediment flux and bed character throughout the seaward reach to analysis, the general patterns show aggre- of the Song Hau distributary. Within this reach, the hydrodynamic regime changes from acting gated sediment in the tidal river upstream of mainly as a tidal river under (a) high-flow conditions, with estuarine processes only penetrating into the zone of estuarine influence where aggre- the distributary channels during part of the tidal cycle, and (b) low-flow conditions, with a partially gation is predicted in association with the intro- mixed estuary moving up and down the distributary channels at all tidal phases. The channel bed duction of salts in the fluid. The effective den- character reflects the convergent sediment fluxes within the tidal river, interface, and estuarine envi- sity of the particles (dashed lines) suggests that, ronments with a seasonal muddy drape covering relict or sand field deposits. This drape is repre- during low-flow conditions, most of the >40 μm sented in (c), which is a conceptual section of the lower distributary channel during low flow and particles are aggregates of relatively low den- which emphasizes the interface that occurs within the tidal river. sity. From McLachlan et al. (in press)

28 Oceanography | Vol.30, No.3 BOX 1. SAMPLING ON THE LOWER MEKONG RIVER

Methodologies used for the Mekong Tropical (see Stephens et al., in press, for methods). were calibrated with in situ water samples, Delta Study combined tools designed for Multiple cast profiles were then integrated which were filtered for mass concentration. both fluvial and marine research. The lower over depth and across the river channel to These data were integrated with ADCP data, Song Hau is an extremely active transporta- arrive at a suspended sand (mode) flux. and spatially variable fluxes were evaluated tion corridor. Boat traffic ranges from small Total suspended sediment fluxes were over different phases of the tide. Cross- individual fishing shells to large commercial evaluated at all seasonal conditions in the sectional discharge values were obtained container ships that continuously transit the 2014–2015 experiments, as well as during by prescribing a no-slip condition at the bot- waterways and thus make sampling on a rigid preliminary field efforts in 2012–2013, using tom, fitting a shape-preserving spline to the time schedule nearly impossible. We used profiles of optical backscatter and ADCP tran- empty regions, and net discharge evaluated a local boat, M/V Mahogany Star, a 20 m sect data (Nowacki et al., 2015; McLachlan by integrating time varying cross-sectional wooden vessel usually employed for tourism et al., in press). The backscatter profiles values over the 24.8-hour tidal cycle. in the upstream distributaries. Our sampling strategy was to occupy transects at locations marked in Figure 1 for 24.8 hours (12.4 hours a when tidal inequalities were at their minimum), and to transit continuously back and forth to develop sequential cross sections of physical water-column properties and sediment suspension. An acoustic Doppler current profiler (ADCP) ran continuously, and on every other crossing, we deployed a con- ductivity, temperature, depth, and turbidity profiler at two to four stations located across the transect. A laser in situ scatterometry and transmissometry (LISST) instrument was also attached to the profiling instrument package to provide information on particle size. At the four phases of tidal currents (max ebb, slack low, max flood, and slack high), water samples were collected using an isokinetic sampler (a fluvial tool) and a snapping Niskin- style bottle sampler (a coastal marine tool). The degree of aggregation was evaluated with a size-settling chamber. During these transects, a pole-mounted multibeam system (a 400 kHz broadband echosounder) linked to a LiDAR system continuously mapped bathymetry and bankline c b morphology using motion correction. After multibeam and LiDAR soundings under- went quality control, and motion sensor and navigation data were merged, bathymetry and elevation grids of 1 × 1 m were created (Allison et al., in press). Suspended sand fluxes were evaluated using data from LISST profiler casts during high discharge in September–October 2014. This volume concentration of a sand- size mode measured by the LISST was cor- rected for the presence of sand-size flocs FIGURE B1. (a) M/V Mahogany Star was used to conduct channel studies during the 2014–2015 using the isokinetic samples and converted international study of the Mekong Delta. This image shows the pole-mounted ADCP and multibeam to a mass concentration of suspended sand systems. (b) Some of the instruments used to evaluate the mud- and sand-sized sediment fluxes within the tidal river and estuary. The small CTD, the OBS (turbidity sensor), and the laser in situ scatterometry and transmissometry (LISST) profiling system are mounted together in a frame, and the isokinetic sampler is at right. (c) Some of the scientists and crew involved in the 2014–2015 studies are shown here gathered on the upper deck of M/V Mahogany Star.

Oceanography | September 2017 29 river. Bedload was not a large contrib- Morphology of Distributary marine influences are both important to utor to net sand fluxes through the sea- Channels the dynamics. The channel floor proved to ward reach of the tidal river, as evaluated Along the fluvial to tidal river to marine be composed of three main channel bed by the translation of bedforms both in continuum, tidally modulated flows, tid- facies—sandy dune fields, low-discharge the upstream and downstream direction ally reversing flows, and/or estuarine deposits of soft muds, and substratum over the tidal cycle. Although the mag- dynamics affect the stresses at the bed outcrops of relict facies (Figure 7; Allison nitude of net bedload flux is not great, and throughout the water column. Thus, et al., in press). Sandy bottoms, character- sand that moves as bedload and main- they control the suspension of mud- and ized in multibeam imagery by the presence tains the observed bedform fields is easily sand-sized sediment and the transport of of dunes, comprise ~19% of the channel drawn out of the bedload layer and main- bedload, which results in differing chan- floor during high discharge. These long tained in suspension at slightly higher nel morphologies. For example, a study ribbon-like deposits occur only in the shear stresses. Therefore, the bedload is by Gugliotta et al. (in press) defines the cross-channel direction, are usually found a critical connection between the sand varying morphologies along the length on the margins of the thalweg (the path source at the channel bed and the net of the Mekong tidal river. Channels in of the deepest points within a channel), export of sand. the landward (non-reversing) tidal river and are generally composed of deposits In contrast to the mud-sized sediment exhibit downstream deepening, fining less than a few meters thick, except as bar fluxes within the Mekong distributar- of bed sand sizes, and limited mud. extensions of mid-channel islands that are ies, sand may not re-enter the distribu- Channels in the seaward reaches of the a focus of sand mining activities. Above tary channels due to: (1) estuarine pro- tidal river exhibit downstream shallow- the island of Cu Lao Dung, which formed cesses that reduce bed shear stresses in ing, uniform sand sizes, and a significant at the bifurcation of the Song Hau, more the denser, more saline water layers, and proportion of mud. Large rivers, such as sandy substrate is found in shallower sec- (2) depositional processes that mantle the the Mekong with its long tidally influ- ondary channels of the main stem that channel bed with mud, cutting off a sand enced reach, allow us to investigate the parallel the deeper navigation channel source to the water column. The tempo- range of along-channel variations in envi- (see Allison et al., 2017, in this issue). By ral sequencing of seasonal sand and mud ronments within the tidal river. Our stud- comparing flow rates from modeling stud- fluxes to the coastal environment likely ies as part of the international collabora- ies (Xing et al., in press), we deduce that affects the development of the shore- tions in 2014–2015 explored the seaward these secondary channels have weaker line environments (see Mullarney et al., reaches of the tidal river to investigate the residual flows that result in a reduction 2017, and Fagherazzi et al., 2017, both in connections between seabed morphol- of sand throughput and a focus area for this issue) and the subaqueous delta and ogy, active transport, and net export of sand storage. As flow in the Song Hau sep- stratigraphic structures found there (see sand and mud. arates into the Dinh An and Tran De sub- Nittrouer et al., 2017, and Liu et al., 2017, We examined the Song Hau distrib- channels around Cu Lao Dung, a larger both in this issue). utary environment where fluvial and area of the bed consists of sand along the more northerly Dinh An pathway to the a ocean. This makes it the dominant path- 1,600 way for the total load of sediment, and the Transect C

) 1,400 b –1

–1 Transect B preferred channel for the 6.5 Mt yr esti- s 1,200 Transect B' Suspended mated net sand export. Transect A Transect Sand Flux 1,000 –1 Transect B' (neap tide) (ton d ) The majority (~80%) of the Song Hau 800 Transect A (neap tide) C 29 x 103 channel floor in the study reach has lit- 600 B 22 x 103 tle or no modern sediment, particularly 400 B’ 2 x 103 during the high-discharge season, result- 3 200 A 29 x 10 ing in widespread evidence of substratum Suspended Sand Flux (kg 0 outcrops. These exposures take the form –200 –20,000 –10,000 0 10,000 20,000 30,000 of steep-sided tabular and sidewall out- Discharge (m3 s–1) crops of resistant strata and furrowed FIGURE 6. (a) Suspended sand flux in the Song Hau distributary channel as evaluated from laser pavements (Figure 7) that are laterally in situ scatterometry and transmissometry profiler casts during high discharge in September– October 2014. The flux is plotted against the tidally varying river discharge (positive downstream, continuous for kilometers along chan- negative upstream) measured over cross sections using a boat-mounted ADCP (see Box 1) The nel. The furrows are composed of grooves results show the strong relationship between discharge and the mass of sand transported in sus- incised into the substratum (and possibly pension, indicating a local bottom source of resuspendable sand. Note the larger water discharges 3 –1 thin modern mud cover) by directionally (up to ~25,000 m s ) during ebbing tide when the river flow is additive with the tide. (b) The tidally averaged flux values at each transect during high river discharge. stable and intermittent tidal currents. The

30 Oceanography | Vol.30, No.3 extent of substratum exposure is greater transportation in the delta. The expand- processes make extrapolation into the than that observed in other systems ing human footprint on the distributar- future difficult. In one scenario, we might such as the Mississippi River, which has ies, as well as the likely alteration to the expect that sediment starvation and a microtidal range. The exposure in the hydrograph and sediment delivery from removal, as well as rising relative sea lev- Song Hau may relate to a relatively lim- the catchment (Allison et al., 2017, in els, will result in channel deepening by ited sand supply from the catchment and/ this issue), means the future evolution of incision into deeper and older strata. This or efficient transfer of both sand and mud these channels will likely differ from their morphodynamic evolution of the chan- through the channels of the tidal river and evolution during the last 3,500 years. nels likely would feedback into the timing estuarine reach. Comparison with stra- Beginning ~1,300–1,400 years ago, the and magnitude of sediment supplied to tigraphy in Mekong Delta borings col- delta front has prograded while the the shoreline (mangrove and other) and lected adjacent to the Song Hau (Ta et al., channel location remained stable but the subaqueous delta. But other dynam- 2002) suggests these substratum out- extended through channel incision into ical factors such as vertical mixing and crops are mainly composed of delta-front late Holocene fluvio-deltaic deposits settling velocities are also at play; how and intertidal-to-subtidal-flat silty sands (e.g., Tamura et al., 2012). Competing might these play out in combination? and sandy silts deposited since the delta began prograding seaward ~3,500 years ago. Erosion and degradation of these a outcrops are apparent at present and are predicted to accelerate in the future due LIDAR to relative sea level rise, sand mining, and BANKLINE dam-induced reduction in sediment supply from the catchment (Allison et al., 2017, in this issue). Thus, these outcrops may provide a source for sand to the lit- toral sand dispersal system and maintain the structure of the subaqueous delta

into the future. ) 2 Modern soft mud layers cover large

parts of the channel beds seasonally Depth (m –20 throughout the interface and estuarine 100 Meters regions of the tidal river (Figure 8). Mud layers are observed to form on the channel floor during the low-discharge season in b deposits 0.25–1.0 m thick in the Dinh An LID Bankline AR BANK and Tran De subchannels surrounding Substratum

Cu Lao Dung. These layers mantle both LI N substratum outcrops (e.g., furrows) and E much of the exposed sand field; their role Dunes in the sand and mud dynamics is dis- cussed above. Large areas of modern mud cover the Tran De beds and are indicative of the enhanced fine-grained sediment trapping and deposition there. Tabular The Song Hau and the other distrib- Substratum utary channels of the Mekong River are ) 2 vital as navigation arteries for transport- ing goods to and from the rapidly grow- 100 Meters Depth (m –20 ing cities in the delta, and they are a vital source of sand for construction purposes FIGURE 7. (a) Multibeam seafloor mosaic at Transect A in Figure 1 (Dinh An subchan- (Brunier et al., 2014). These channels now nel) in the 2014 high flow study showing longitudinal furrows along the left descend- ing distributary channel bank. (b) Multibeam mosaic of Transect B in Figure 1 (also also serve as trunks for a vast array of arti- Dinh An subchannel, upstream of Transect A) in the high flow study showing a tabu- ficial side channels used for irrigation and lar bankline outcrop and mid-channel dunes. After Allison et al. (in press)

Oceanography | September 2017 31 High Flow (2014)

Meters 0 100 200

Low Flow (2015)

Meters 0 100 200 FIGURE 8. Multibeam bed surveys illustrating the change in channel beds between seasons. Under high-flow conditions in 2014, sand dunes are evident (see red circle) and bed-surface sediment samples show a coarse sand bed. Under low-flow conditions in 2015, no bed features are evident in the repeat survey (see red circle), and surface samples of soft mud were obtained.

MEKONG DISTRIBUTARY mixing dynamics suggests that, for SUMMARY EVOLUTION these types of large deltaic channels, the The collaborative Mekong Tropical Delta The sum of sediment fluxes from active sequence of bifurcation, enhanced mix- Study conducted in the seaward reach sedimentary processes throughout the ing, sediment trapping in the subchannel of the Mekong tidal river provides some tidal river-to-estuary system leads to sta- with lesser flow, and eventual abandon- of the most comprehensive data on this bility or instability of channel morpholo- ment should be expected. Abandonment portion of the river to date. We gained gies on a range of temporal scales. On the of the Tran De subchannel and focusing new insights into the modulation of flu- shortest seasonal scale, mud mantling of of freshwater discharge in the Dinh An vial source signals in tidal rivers and the estuary/interface regions shuts down subchannel would likely result in a more estuaries, insights that suggest signifi- sand transport seasonally. On a decadal stratified estuary. cant impact on sediment signals reaching scale, the demise of the Tran De sub- However, erosion of substratum and the coastal ocean. The integrated studies channel seems likely because its flows are deepening of channels as sea level rise of hydrodynamics, sediment transport weak, and its channel depth is shallow. accelerates would compete with the mix- and deposition, and channel morphol- The shallower depth leads to a more ver- ing and settling processes. Each distrib- ogy shed new light on: (1) the impact of tically mixed water column, and during utary will be affected by a different com- tidally variable discharge and bed stress estuarine conditions promotes further bination of these impacts, resulting in on sediment pathways and total sedi- aggregation of organic-rich particles. different winners and losers. Although ment flux, (2) the interaction of mud and Rapid settling ensues, and sediment trap- we are uncertain about how the balance sand through depositional processes at ping further promotes muddy sediment will play out, we know that distributaries the seabed, and (3) the role that mixing deposition and mud mantling of any sand are becoming less stable, as has happened plays in particle settling within the tidal beds that exist, as is evident in multibeam in the Ba Lai distributary to the north river interface and estuary, with feed- surveys. Historical aerial imagery also (Tamura et al., 2012), and channel domi- backs through channel depth to chok- suggests the Tran De subchannel has been nance may shift. Hence, the distributary- ing of a bifurcated distributary. In sum, evolving into a less active transport path- channel network in the delta, which in the sediment source signal that leaves the way to the coastal ocean (Nowacki et al., general has been stable over thousands purely fluvial end member of the tidal 2015). This feedback between reduced of years, is likely to evolve toward a new river is dramatically altered as it transits flow in a subchannel and subsequent equilibrium state. through the tidal river and estuary, a zone

32 Oceanography | Vol.30, No.3 where fluvial process dominance yields to McLachlan, R.L., A.S. Ogston, and M.A. Allison. In Xing, F., E.A. Meselhe, M.A. Allison, and press. Implications of tidally-varying bed stress and H.D. Weathers III. In press. Analysis and numer- tidal process dominance before the river intermittent estuarine stratification on fine-sediment ical modeling of the flow and sand dynamics waters reach the coastal ocean. dynamics through the Mekong’s tidal river to in the lower Song Hau channel, Mekong Delta. estuarine reach. Continental Shelf Research, Continental Shelf Research. REFERENCES https://doi.org/10.1016/j.csr.2017.07.014. Mekong River Commission. 2005. Overview of the Allison, M.A., C.A. Nittrouer, A.S. Ogston, ACKNOWLEDGMENTS Hydrology of the Mekong Basin. Mekong River J.C. Mullarney, and T.T. Nguyen. 2017. This research was funded by the US Office of Naval Commission, Vientiane Lao PDR. 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