SEASONAL IMPACTS ON SEDIMENT CHARACTERISTICS IN THE GANGES-BRAHMAPUTRA-MEGHNA DELTA

The Ganges-Brahmaputra-Meghna (GBM) Delta is one of the largest deltas in the world, covering more than 100,000 km². The GBM River systems transport over one billion tons of sediment per year and deliver ~750 million tons to the Bay of Bengal, of which, ~25% is tidally reworked into the abandoned western delta plain. The Sundarbans National Forest (SNF) is located within the GBM Delta and, covering more than 10,000 km², is the world’s largest continuous mangrove stand. Due to this delivery of sediment, the SNF platform elevation currently keeps pace with regional sea level rise, but India’s National River Linking Project (NRLP) could decrease suspended sediment by 39-75% and 9-25% in the Ganges and Brahmaputra Rivers, respectively. In this study, we examine the idea that sediment in the tidal channels is constantly resuspended throughout the year and has the ability to be delivered to the mangrove platform during the dry season when sediment input to the system is at a minimum. Over two field seasons, 70 shallow (< 50 cm) sediment cores were collected, imaged using x-radiography to observe fine scale changes in depositional characteristics on tidal channel banks, and subsampled for subsequent textural analysis. In more than half of the cores examined, there is no change in the nature of the laminations, suggesting continuous depositional processes throughout the year. More than half of the cores show laminations extending deeper in the dry season than the monsoon season. Over 70% of cores showed surface (0-2 cm) grain sizes increase between the monsoon to the dry season, which could be the product of the winnowing of fine sediments as overall sediment supply decreases. We are continuing to look at spatial variability to understand the physical processes controlling depositional patterns can change across scales.