Process regime, salinity, morphological, and sedimentary trends along the fluvial to marine transition zone of the mixed-energy Mekong River delta, Vietnam

Marcello Gugliotta, Yoshiki Saito, Van Lap Nguyen, Thi Kim Oanh Ta, Rei Nakashima, Toru Tamura, Katsuto Uehara, Kota Katsuki, Seiichiro Yamamoto

Research output: Contribution to journalArticle

32 Citations (Scopus)

Abstract

The fluvial to marine transition zone (FMTZ) is the area of coastal rivers in which sedimentation is controlled by the interaction of fluvial and marine processes. This study examines the FMTZ of the Mekong River delta, along a total channel length of ~660 km. Methods consist of collection and analysis of channel bed sediment samples, measurements of channel morphological parameters, and recognition of mangrove, molluscan, and diatom species. The process regime, salinity, morphological, and sedimentary trends recognized were used to define two main tracts for this FMTZ: an upstream, fluvial-dominated tract and a downstream, tide-dominated tract. In more detail, they allow the identification of four subzones, from upstream to downstream: 1) fluvial-dominated, tide-affected; 2) fluvial-dominated, tide-influenced; 3) tide-dominated, fluvial-influenced; and 4) tide-dominated, fluvial-affected. Tide-induced water-level changes affect the entire study area and extend into Cambodia. Measured salinity intrusion extends ~15 km upstream of the river mouth during wet season, and ~50 km during dry season. Brackish water species of mangroves, mollusks, and diatoms, however, occur landward of these limits, suggesting that highly diluted brackish water may reach ~160 km upstream of the river mouth during the dry season. In the fluvial-dominated tract, channels are sinuous and show a seaward-deepening trend, whereas width is relatively constant. In the tide-dominated tract, channels are straight, and show seaward-widening and seaward-shallowing trends. Natural levees are present in the fluvial-dominated, tide-affected subzone, but are replaced by mangroves elsewhere along the FMTZ. In the fluvial-dominated tract, mud content is low, sand grain size fines seaward, and gravelly sand and sand are the dominant facies. In the tide-dominated tract, mud content is high, sand grain size is constant, recycled sand is common, and tidal rhythmites are the dominant facies. Mud pebbles are common in sediments throughout a large part of the FMTZ. These trends characterizing the FMTZ of the Mekong River delta seem to be present in other systems and likely represent a general FMTZ pattern. Nonetheless, minor differences may be observed between different types of systems, or because of differences in local conditions. The comprehensive description of trends and their mutual relationships along the FMTZ presented herein provides critical information that can form the basis of a general conceptual model and can help to better understand these complex zones.

Original languageEnglish
Pages (from-to)7-26
Number of pages20
JournalContinental Shelf Research
Volume147
DOIs
Publication statusPublished - Sep 1 2017

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Mekong River
Vietnam
transition zone
tides
tide
salinity
energy
river
sand
mangrove
mud
Bacillariophyceae
brackish water
rivers
dry season
mouth
diatom
grain size
rhythmite
trend

All Science Journal Classification (ASJC) codes

  • Oceanography
  • Aquatic Science
  • Geology

Cite this

Process regime, salinity, morphological, and sedimentary trends along the fluvial to marine transition zone of the mixed-energy Mekong River delta, Vietnam. / Gugliotta, Marcello; Saito, Yoshiki; Nguyen, Van Lap; Ta, Thi Kim Oanh; Nakashima, Rei; Tamura, Toru; Uehara, Katsuto; Katsuki, Kota; Yamamoto, Seiichiro.

In: Continental Shelf Research, Vol. 147, 01.09.2017, p. 7-26.

Research output: Contribution to journalArticle

Gugliotta, Marcello ; Saito, Yoshiki ; Nguyen, Van Lap ; Ta, Thi Kim Oanh ; Nakashima, Rei ; Tamura, Toru ; Uehara, Katsuto ; Katsuki, Kota ; Yamamoto, Seiichiro. / Process regime, salinity, morphological, and sedimentary trends along the fluvial to marine transition zone of the mixed-energy Mekong River delta, Vietnam. In: Continental Shelf Research. 2017 ; Vol. 147. pp. 7-26.
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AU - Saito, Yoshiki

AU - Nguyen, Van Lap

AU - Ta, Thi Kim Oanh

AU - Nakashima, Rei

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N2 - The fluvial to marine transition zone (FMTZ) is the area of coastal rivers in which sedimentation is controlled by the interaction of fluvial and marine processes. This study examines the FMTZ of the Mekong River delta, along a total channel length of ~660 km. Methods consist of collection and analysis of channel bed sediment samples, measurements of channel morphological parameters, and recognition of mangrove, molluscan, and diatom species. The process regime, salinity, morphological, and sedimentary trends recognized were used to define two main tracts for this FMTZ: an upstream, fluvial-dominated tract and a downstream, tide-dominated tract. In more detail, they allow the identification of four subzones, from upstream to downstream: 1) fluvial-dominated, tide-affected; 2) fluvial-dominated, tide-influenced; 3) tide-dominated, fluvial-influenced; and 4) tide-dominated, fluvial-affected. Tide-induced water-level changes affect the entire study area and extend into Cambodia. Measured salinity intrusion extends ~15 km upstream of the river mouth during wet season, and ~50 km during dry season. Brackish water species of mangroves, mollusks, and diatoms, however, occur landward of these limits, suggesting that highly diluted brackish water may reach ~160 km upstream of the river mouth during the dry season. In the fluvial-dominated tract, channels are sinuous and show a seaward-deepening trend, whereas width is relatively constant. In the tide-dominated tract, channels are straight, and show seaward-widening and seaward-shallowing trends. Natural levees are present in the fluvial-dominated, tide-affected subzone, but are replaced by mangroves elsewhere along the FMTZ. In the fluvial-dominated tract, mud content is low, sand grain size fines seaward, and gravelly sand and sand are the dominant facies. In the tide-dominated tract, mud content is high, sand grain size is constant, recycled sand is common, and tidal rhythmites are the dominant facies. Mud pebbles are common in sediments throughout a large part of the FMTZ. These trends characterizing the FMTZ of the Mekong River delta seem to be present in other systems and likely represent a general FMTZ pattern. Nonetheless, minor differences may be observed between different types of systems, or because of differences in local conditions. The comprehensive description of trends and their mutual relationships along the FMTZ presented herein provides critical information that can form the basis of a general conceptual model and can help to better understand these complex zones.

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