Coupled Landscape and Channel Dynamics Across the Ganges-Brahmaputra Tidal-Fluvial Continuum, Southwest Bangladesh

The Ganges-Brahmaputra (G-B) Delta of Bangladesh and India is widely considered to be one of the most vulnerable coastal systems in the world, owing to locally-accelerated sea level rise and landscape modification by human activities. Recent research efforts have identified pronounced elevation differences between natural and embanked areas in the G-B tidal delta plain, suggesting that natural landscapes are keeping pace with sea-level rise while anthropogenic locations are actively subsiding. However, this observation represents a singular point in time, and longitudinal trends of surface elevation in these regions are presently unknown. For communities residing in upstream regions of G-B delta plain, river bank stability is a primary concern, yet no scientifically-grounded assessments of channel dynamics exist for these areas. To address these knowledge deficits, this dissertation aims to: (1) compare rates of surface elevation change in the natural G-B tidal delta plain to those of current and projected trends of sea-level rise; (2) identify subsurface processes that influence elevation change, constrain their relative importance, and place findings within the context of carbon storage; and (3) develop a process-based framework for predicting riparian substrate composition across the tidal-fluvial transition zone of the G-B delta plain. Taken together, it is hoped that understanding the controls on elevation change and substrate erodibility will contribute to more effective management and adaptation strategies in these regions and in other deltaic systems worldwide. The results of the studies indicate that: (1) natural areas of the G-B tidal delta plain are currently maintaining surface equilibrium with accelerated sea-level rise, though questions remain about the long-term future of this region, especially with regard to the continued supply of upstream sediments; (2) both physical and biotic parameters govern subsurface dynamics and elevation change in the G-B tidal delta plain, and the soils of this region host larger amounts of terrestrial carbon than expected; and (3) a shift from uniform to non-uniform flow conditions influences the stratigraphic architecture of river banks across the G-B tidal-fluvial transition zone. This change in stratigraphy, in turn, controls river channel mobility and thus susceptibility to fluvial erosion