Recent evolution of the Irrawaddy (Ayeyarwady) Delta and the impacts of anthropogenic activities: A review and remote sensing survey

Intensive studies have been conducted globally in the past decades to understand the evolution of several large deltas. However, despite being one of the largest tropical deltas, the Irrawaddy (Ayeyarwady) Delta has received relatively little attention from the research community. To reduce this knowledge gap, this study aims to provide a comprehensive assessment of the delta's evolution and identify its influencing factors using remote sensing images from 1974 to 2018, published literature and available datasets on the river, and human impacts in its drainage basin. Our results show that 1) Based on the topographic and geomorphological features, the funnel-shaped Irrawaddy Delta can be divided into two parts: the upper fluvial plain and the lower low-lying coastal plain; 2) The past 44-year shoreline changes show that overall accretion of the delta shoreline was at a rate of 10.4 m/year, and approximately 42% of the shoreline was subjected to erosion from 1974 to 2018. In the western coast, 60% of shoreline was under erosion with an average shoreline change rate of 0.1 m/year. In the east part, 81% of the shoreline was accreted with an average accretion rate of 24 m/year; 3) River channel geomorphological analysis indicates that three distributaries of the Irrawaddy, Bogale, and Toe have developed most active sandbars, which coincides with the amount of water they discharged (>50%). This implies that these three distributaries might be the currently most active channels in the delta; 4) The Irrawaddy mainstream in the Central Dry Zone (the original high sediment yield area) has become less braided and some tributaries have become increasingly straightened, which are highly likely related to reductions in sediment supply and peak flow induced by dam construction; 5) The large geomorphological adjustments at the two bifurcation points means that the diversions and fractions of water and sediment into the distributaries have likely already changed due to anthropogenic impacts. Our comprehensive analysis suggests that increasing human activities have caused reductions in coarse sediment supply entering the coastal delta plain, further inducing the erosion of the major channels in the lowermost delta and the western delta coast, and the adjustments of fluvial and coastal geomorphology; meanwhile, deforestation and terrestrial mining have provided extra fine sediment, which is mainly transported by the monsoon-driven current to the eastern coast to in part maintain its rapid accretion. Given the situation of rapidly increasing population and climate change, the current natural equilibrium state of the delta setting will most likely be disturbed in the near future. Therefore, our work calls for more intensive monitoring- and modeling-based study in order to better understand the controlling factors influencing the delta evolution in the future.