Abstract 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.

中文翻译：

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伊洛瓦底江（伊洛瓦底江）三角洲的近期演变和人类活动的影响：综述和遥感调查

摘要 在过去的几十年中，全球进行了深入的研究，以了解几个大型三角洲的演变。然而，尽管伊洛瓦底江三角洲是最大的热带三角洲之一，但研究界的关注相对较少。为了缩小这一知识差距，本研究旨在利用 1974 年至 2018 年的遥感图像、已发表的关于河流的文献和可用数据集以及人类对流域的影响，对三角洲的演变进行全面评估并确定其影响因素。我们的研究结果表明： 1）根据地形地貌特征，漏斗状的伊洛瓦底江三角洲可分为两部分：上部河流平原和下部低洼沿海平原；2）过去44年的海岸线变化表明，三角洲海岸线总体以10.4 m/年的速度增加，1974-2018年约42%的海岸线受到侵蚀。在西海岸，60%的海岸线受到侵蚀，平均海岸线变化率为 0.1 m/年。在东部，81%的海岸线被增生，平均增生速度为24 m/年；3）河道地貌分析表明，伊洛瓦底江、博加莱和托厄三个支流发育最活跃的沙洲，与其排放的水量相吻合（>50%）。这意味着这三个支流可能是三角洲中目前最活跃的渠道；4）中部干旱区（原高产沙区）伊洛瓦底江干流的辫状变少，部分支流变得越来越直，这极有可能与大坝建设导致输沙量减少和峰值流量减少有关；5) 两个分岔点的大地貌调整意味着水和沉积物进入支流的分流和比例可能已经因人为影响而发生变化。综合分析表明，随着人类活动的增加，进入滨海三角洲平原的粗泥沙供应减少，进一步导致三角洲最下部和三角洲西部海岸主要河道的侵蚀，以及河流和海岸地貌的调整；同时，森林砍伐和陆地采矿提供了极细的沉积物，这些沉积物主要由季风驱动的洋流输送到东部海岸，以部分保持其快速堆积。鉴于人口快速增长和气候变化的情况，三角洲环境目前的自然平衡状态很可能在不久的将来被打破。因此，我们的工作需要更深入的基于监测和建模的研究，以便更好地了解影响未来三角洲演化的控制因素。三角洲环境当前的自然平衡状态很可能在不久的将来受到干扰。因此，我们的工作需要更深入的基于监测和建模的研究，以便更好地了解影响未来三角洲演化的控制因素。三角洲环境当前的自然平衡状态很可能在不久的将来受到干扰。因此，我们的工作需要更深入的基于监测和建模的研究，以便更好地了解影响未来三角洲演化的控制因素。