Mid-channels Islands as Dynamic Sediment Traps: Sediment Dynamics and Morphologic Evolution of the Mangrove-forested Meinmahla Island, Ayeyarwady Delta, Myanmar

Vegetated mid-channel islands play an important role in modulating fluxes of sediment, carbon, and nutrients through the tidally-influenced region of river deltas. Yet, the hydrodynamics, sediment dynamics, and morphologic evolution of these features are poorly understood, limiting our ability to accurately parameterize models and predict the stability of vulnerable deltas. Meinmahla Island is a relatively unaltered mangrove-forest preserve in the mouth of the Bogale River, a distributary of the Ayeyarwady Delta, Myanmar. Field measurements of morphology, hydro- and sediment-dynamics in 20172019 provide insight into the fundamental processes governing the evolution of this mid-channel island. Water depth, salinity, and turbidity were monitored semi-continuously, and Aquadopp velocity profilers with turbidity and salinity sensors were deployed seasonally in low-connectivity (dead end) and high-connectivity channels of the island. The morphologic evolution was evaluated using grain size, 210Pb geochronology in 1-m sediment cores, aerial imagery, and surveys of channel networks. Ebb-dominant, low-connectivity channels along the island exterior import 0.982.94 kg/s of sediment year-round to the land surface, or 0.141.5% of the Bogale River sediment export. These exterior, low-connectivity channels provide a fraction of the sediment needed to maintain the observed 0.78 cm/yr accretion rate, and most of the sediment delivery occurs via interior, high-connectivity channels. Interior channels retain water masses that are physically distinct from the water in the Bogale River, and estuarine processes at the channel mouths actively import sediment into the island. Sediment is sourced to the island from upriver in the wet season and offshore in the dry season, as the Bogale River estuary shifts seasonally with river discharge. The interior channel networks have remained remarkably stable while the island has aggraded and prograded. However, the high-connectivity channel is responding to a drainage network change by narrowing and shoaling in its middle over decadal timescales. Overall, mid-channel islands are active filters for sediment and nutrients at the river-ocean interface, and these resilient landscape features respond dynamically to changes in interior network connectivity.