CONTROLS ON THE DOWNSTREAM VARIABILITY OF CHANNEL MORPHODYNAMICS AND POINT-BAR ARCHITECTURE IN A TIDE-INFLUENCED MONSOON RIVER: SITTAUNG RIVER, MYANMAR (Invited Presentation)

Characterization of the meandering river deposits is challenging as the interplay between fluvial and tidal processes results in a complex point-bar architecture that varies considerably along proximal-distal trends in the fluvial-marine transition zone. Despite recent advances in sedimentological research of the transitional environment, still little is known about what controls downstream variability of channel morphodynamics and resultant point-bar architecture along the transition zone. Sittaung River, flowing south into the Gulf of Martaban, is a meandering river with large meander belt width and a 150 km long transition zone. In a fluvially dominated zone, channel migrates mainly by expansion and downstream rotation (10~100 m/yr). The presence of resistant substrates such as river valley rocks and mud-filled oxbow lakes leads to downstream translation and tortuous bend morphology. Point bars exhibit a distinct downstream fining trend around the meander bend, and consist of seasonal inclined heterolithic stratification (IHS). With increased tidal influence toward downstream, channels continue to migrate at much slower rate than upstream and locally by lateral extension (10~30 m/yr). Increased tidal prism and channel-confinement by resistant substrates allows channel to maintain fast migration rate despite high channel sinuosity. Point bars display a less distinct downstream fining trend and consist of seasonal IHS with increasing tidal signatures. In a fluvially influenced, tidal dominated zone, channel width increases exponentially to accommodate increased tidal prism. Channels migrate mainly downstream at much reduced rate (<1 m/year). Locally, rapid channel migration (700~900 m/yr) occurs in the tide-dominated zone thanks to immense tidal prism and sediment-laden flows during peak river flood, which is in contrast with the least active channel morphodynamics known from the tidal backwater zone of major rivers. Point bars consist of predominantly tidal IHS influenced by river floods. Sittaung River showcased the complexity and variability of downstream controls on the channel morphodynamics and resultant point-bar architecture along the fluvial-marine transition zone with distinct monsoon discharge, macrotidal regime, seasonal sediment-laden flows and resistant substrates.