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Vegetated mid‐channel islands play an important though poorly understood role in the sediment dynamics and morphology of tide‐dominated deltas. Meinmahla Island is a mangrove‐forest preserve at the mouth of the Bogale distributary channel, in the Ayeyarwady Delta, Myanmar. In this relatively unaltered mid‐channel island, sediment dynamics can be directly connected to morphology. Field measurements from 2017 to 2019 provide insight into the pathways for sediment transport and resulting morphological evolution. Water depth, salinity and turbidity were monitored semi‐continuously, and velocity profilers with turbidity and salinity sensors were deployed seasonally in single‐entrance (dead‐end/blind) and multi‐entrance tidal channels of the island. The morphological evolution was evaluated using grain size, 210Pb geochronology, remote sensing and channel surveys. The data show that ebb‐dominant, single‐entrance channels along the island exterior import sediment year‐round to the land surface. However, these exterior channels do not deliver enough sediment to maintain the observed ca 0.8 cm/yr accretion rate, and most of the sediment import occurs via interior, multi‐entrance channels. Interior channels retain water masses that are physically distinct from the water in the Bogale distributary, and estuarine processes at the tidal‐channel mouths import sediment into the island. Sediment is sourced to the island from upriver in the wet season and from the Gulf of Mottoma in the dry season, as the location of the estuary shifts seasonally within the Bogale distributary. The salinity and biogeochemistry of the distributary water are affected by interactions with sediment and groundwater in the island interior. The largest interior channels have remained remarkably stable while the island has aggraded and prograded over decadal timescales. However, the studied multi‐entrance channel is responding to a drainage‐network change by narrowing and shoaling. Overall, mid‐channel islands trap sediment and associated nutrients at the river–ocean interface, and these resilient landscape features evolve in response to changes in drainage‐network connectivity. [ABSTRACT FROM AUTHOR]

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]