EFFECT OF SHIFTING HYDROLOGIC REGIMES ON THE BENGAL BASIN AND GANGES-BRAHMAPUTRA RIVER DELTA SEQUENCE: MONSOON VARIABILITY AND LAKE-BURST FLOODS IN THE LATE QUATERNARY (Invited Presentation)

The modern Ganges-Brahmaputra River is among the world’s largest fluvial systems, with a catchment of 1.6 million km² and 1200 km³/yr of water discharge. Yet the river is almost singularly driven by the Indian Summer Monsoon, with 80% of water discharge and 95% of sediment load occurring during the roughly 5-month monsoon season. The system is thus highly sensitive to monsoon strength, with variance in river discharge up to 50% over annual to century timescales, and even greater variance up to 1000% over multi-millennial timescales. This latter, long-term variability is a function of both regional and global boundary conditions (e.g., insolation, temperature, ice volume), and thus partly phases with glacioeustatic sea level – another major control on fluviodeltaic margin sequences. During the last glacial period, though, the weak monsoon coupled with enhanced glacier activity led to a major influence of paleoflood hydrology on the fluvial transport system. Specifically glacial-lake outburst floods originating in the Tsangpo valley of Tibet, some of megaflood scale (>10⁶ m³/s), were regularly routed through the Bengal basin via the Brahmaputra river valley. This talk will explore the impact of these shifting hydrologic regimes on geomorphic and stratigraphic development in the Bengal basin and Ganges-Brahmaputra delta system.