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The largest deltas on Earth are tide‐dominated with multiple distributaries, which can experience different forcings. Yet, most observational studies of sediment dynamics in these systems focus on a single distributary. Comparison of the tidal‐to‐estuarine reaches of three representative distributaries of the Ayeyarwady Delta, Myanmar highlights the variability in sediment retention and export within a megadelta. The Ayeyarwady River has no mainstem dams, providing an opportunity to examine a relatively natural, monsoon‐dominated system. Observations were collected during high flow (September 2017) and low flow (March 2018 and 2019). The eastern Yangon distributary empties into the sheltered Gulf of Mottoma while the central Bogale and western Pathein distributaries are exposed to seasonal waves. During high flow, suspended‐sediment concentration (SSC) was consistent across all three distributaries (0.2–0.4 g/L). The upper delta retained 20%–60% of the mainstem sediment load, while the lower distributaries were tidal rivers exporting sediment. During low flow, the lower distributaries were partially mixed estuaries with little net discharge. However, SSC increased to >1 g/L in the Yangon distributary, remained constant in the Bogale distributary, and decreased in the Pathein distributary compared to high flow. This difference was driven by offshore sediment supply; fine sediment is retained near the mouth of the Yangon and advected away from the Pathein distributary. Consequently, nearshore sediment residence time in monsoon‐influenced megadeltas is controlled by coastal orientation, the timing of discharge with respect to ocean conditions, and tidal amplification. Changes to fluvial sediment delivery can cause non‐uniform coastal impacts due to these differing distributary processes. Key Points: Individual distributaries of tropical megadeltas can exhibit significantly different hydrodynamics and sediment transport processesIn the Ayeyarwady Delta, low‐flow estuarine processes import offshore sediment into distributaries when offshore sediment is available and resuspendedSediment residence time in the nearshore is controlled by tidal amplification and the coastal orientation with respect to monsoon winds [ABSTRACT FROM AUTHOR]

The Ayeyawady delta is the last Asian megadelta whose evolution has remained essentially unexplored so far. Unlike most other deltas across the world, the Ayeyawady has not yet been affected by dam construction, providing a unique view on largely natural deltaic processes benefiting from abundant sediment loads affected by tectonics and monsoon hydroclimate. To alleviate the information gap and provide a baseline for future work, here we provide a first model for the Holocene development of this megadelta based on drill core sediments collected in 2016 and 2017, dated with radiocarbon and optically stimulated luminescence, together with a reevaluation of published maps, charts and scientific literature. Altogether, these data indicate that Ayeyawady is a mud-dominated delta with tidal and wave influences. The sediment-rich Ayeyawady River built meander belt alluvial ridges with avulsive characters. A more advanced coast in the western half of the delta (i.e., the Pathein lobe) was probably favored by the more western location of the early course of the river. Radiogenic isotopic fingerprinting of the sediment suggests that the Pathein lobe coast does not receive significant sediment from neighboring rivers. However, the eastern region of the delta (i.e., Yangon lobe) is offset inland and extends east into the mudflats of the Sittaung estuary. Wave-built beach ridge construction during the late Holocene, similar to several other deltas across the Indian monsoon domain, suggests a common climatic control on monsoonal delta morphodynamics through variability in discharge, changes in wave climate or both. Correlation of the delta morphological and stratigraphic architecture information on land with the shelf bathymetry, as well as its tectonic, sedimentary and hydrodynamic characteristics, provides insight on the peculiar growth style of the Ayeyawady delta. The offset between the western Pathein lobe and the eastern deltaic coast appears to be driven by tectonic-hydrodynamic feedbacks as the extensionally lowered shelf block of the Gulf of Mottama amplifies tidal currents relative to the western part of the shelf. This situation probably activates a perennial shear front between the two regions that acts as a leaky energy fence. Just as importantly, the strong currents in the Gulf of Mottama act as an offshore-directed tidal pump that helps build the deep mid-shelf Mottama clinoform with mixed sediments from the Ayeyawady, Sittaung and Thanlwin rivers. The highly energetic tidal, wind and wave regime of the northern Andaman Sea thus exports most sediment offshore despite the large load of the Ayeyawady River. [ABSTRACT FROM AUTHOR]

As the Ayeyarwady River approaches the Andaman Sea, it splits into numerous delta distributary channels, which together discharge >108 t/y of sediment to the coastal ocean. This study aims to understand: 1) sediment retention and geomorphic variability through the lower tidally influenced distributaries, 2) deltaic growth along mangrove shorelines, and 3) sediment export through this relatively natural, but rapidly changing deltaic system. We focus on three of the distributary channels that have differing fluvial discharge and marine tidal forcing. There is an order-of-magnitude difference between the discharge carried by the largest of the distributaries, the Yangon River, and the smallest studied, the Bogale River. From east to west, the tidal range at the distributary mouths decreases from ~6 m at the eastern Yangon River, to 3.5 m at central Bogale River, to 3.2 m at the western Pathein River. Seasonal variations are associated with tropical monsoons, and our studies occurred during both high- and low-discharge conditions. During high-discharge conditions, suspended-sediment concentrations (SSC) and sediment dynamics are similar across all three distributary channels. They all carry sediment with a median grain size of 3-4 microns in suspension. During low-discharge, salinity intrusion extends many tens of kilometers upstream of the river mouths, shifting the dynamics within the distributaries from those of a tidal river to an estuary. In this regime, SSC values are observed to decrease in the Pathein River (10s mg/L), while in the Yangon River, SSC increases to >5 g/L. In addition to in-situ data collected during the two seasonal conditions, surface SSC derived from calibrated multispectral remote-sensing products provide a synoptic view of turbidity across the delta, and resolve long-term trends in surface SSC for the distributaries of the Ayeyarwady River. Landsat images suggest greater turbidity toward the apex of the Gulf of Mottama, and in all three distributaries, surface SSC is generally greater near the mouths, and decreases upstream, consistent with our observations. Relating suspended sediment transport in the distributaries to local morphology, we find that channel-bed sediment is primarily muddy sand, but mud deposits can be found in regions of the estuarine turbidity maximum. On the adjacent mangrove-forested intertidal surfaces, morphodynamic studies document sediment-accumulation rates of ~1 cm/yr in the aggrading mangrove forest and prograding subaqueous shoal. Natural networks of dendritic tidal channels within the forest efficiently deliver sediment to intertidal land surfaces distal to the main channels. The mangrove forests of the Ayeyarwady Delta, while diminishing in size due to human factors, are an important sediment sink upstream of the coastal ocean. Due to its large sediment discharge, dynamic seasonal and tidal conditions, and intact mangrove forests, the Ayeyarwady Delta is an important and understudied component of global source-to-sink systems. [ABSTRACT FROM AUTHOR]