Presenter: Andrea Ogston (University of Washington)

Description:
Interdistributary islands are ubiquitous features of tide-dominated deltas that are morphologically responsive to fluvial and marine processes. Patterns of sediment deposition and erosion are modified by vegetation changes and other anthropogenic disturbances. Meinmahla Island is an undeveloped mangrove preserve within the Bogale distributary of the Ayeyarwady Delta, and in-situ observations of this  natural site were combined with remote-sensing studies to examine impacts of human alteration on coastal stability. Sediment cores were collected from the prograding front and aggrading tidal flats of the island, and were evaluated by grain-size measurements, x-radiography, and 210-Pb geochronology. Accumulation of fine sand and silt on the seaward fringe of the island at a rate of ~1.3 cm/y corresponds well with remotely sensed shoreline progradation over the past ~70 yrs, while silt and clay accumulates on the adjacent flats at the rate of local sea-level rise. Complex tidal channel networks provide the island interior with water and sediment over tidal and seasonal cycles. Channel density and sinuosity were measured from Landsat and Planetscope imagery. Of all regional land cover types, mangrove forest throughout Meinmahla Island likely exhibits the highest channel density, but mangrove canopies obscure channels at small scales. Natural channel networks are relatively stable over the observation period. Tidal-channel networks in adjacent, deforested islands are infilling and becoming less complex over decadal timescales. Infilling results from changes to hydrodynamic forcing following vegetation change, more direct agricultural practices, or both. The reduction in channel density may result in subsidence, as less sediment can be delivered to interior land surfaces. These combined data sets demonstrate processes causing morphologic changes to interdistributary islands and provide valuable contrast to other global tropical delta studies where human impacts are common.