Comparing Distributary-Channel Hydrodynamics and Sediment Dynamics in a Tide-Dominated, Tropical Delta: The Ayeyarwady River Delta, Myanmar

The Ayeyarwady River, Myanmar, has few dams compared to other Himalaya-sourced rivers, and currently carries the 3rd largest sediment load globally. Consequently, its 250-km-wide, tide-dominated delta provides an opportunity to compare hydrodynamics and sediment dynamics between distributaries and to connect variability with morphology. Processes in the lower reach of three representative distributaries were investigated during the high-flow (September 2017) and low-flow (March 2018 and 2019) seasons. Tidal range decreases across the delta from ~6 m at the mouth of the eastern Yangon distributary, to ~3 m at the central Bogale distributary, to ~2.5 m at the western Pathein distributary. During the high-flow season, the lower distributaries were freshwater tidal channels, and flux decomposition shows that fluvial export dominated sediment transport. The Yangon distributary discharge was an order-of-magnitude larger than that of the Bogale distributary, the smallest of the studied channels. Suspended-sediment concentrations (SSC) were consistent across all three distributaries, and the riverbeds were sandy with ephemeral mud deposits. During the low-flow season, tidal fluxes dominated sediment transport, and there was little to no net discharge from the distributaries. Yet, there was a divergence in turbidity compared to high-flow conditions: SSC increased to >1 g/L in the Yangon distributary, remained constant in the Bogale distributary, and decreased in the Pathein distributary. This divergence was driven by offshore sediment supply, as there is an extensive region of unconsolidated mud near the mouth of the Yangon distributary that can be carried upriver by tidal pumping. Future damming would alter sediment flux most significantly in the Yangon distributary. Decreasing overall discharge and flattening the hydrograph would shift the high-flow tidal river and low-flow, partially-mixed estuary toward salt-wedge stratification. This shift could increase sediment retention, which would be detrimental to navigation. These observations demonstrate that sedimentary processes in individual distributaries cannot be assumed to apply in all distributaries and that damming impacts may not be felt equally across a delta.