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

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How Tidal Processes Impact the Transfer of Sediment from Source to Sink: Mekong River Collaborative Studies OceTHE OFFICIALa MAGAZINEn ogOF THE OCEANOGRAPHYra SOCIETYphy 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 COPYRIGHT This article has been published in Oceanography, Volume 30, Number 3, a quarterly journal of The Oceanography Society. Copyright 2017 by The Oceanography Society. All rights reserved. USAGE Permission is granted to copy this article for use in teaching and research. Republication, systematic reproduction, or collective redistribution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The Oceanography Society. Send all correspondence to: [email protected] or The Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. 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 head waters 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 salt water 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 2 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.
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