Sustained growth of river-mouth bars in the vulnerable Changjiang Delta

摘要

Based on 345 satellite images from 1991 to 2018, we examine the height, area and morphological changes of two large newborn river-mouth bars in the vulnerable Changjiang Delta as well as their influencing factors. The growth of inaccessible river-mouth bars is less affected by human estuarine activities, therefore, studying their growth shows particular promise in revealing the effects of relative sea level rise and human activities upstream of the river basin. We find the morphologic changes of the river-mouth bars in the Changjiang Delta are seriously affected by waves, tides and runoff, but their height and area growth depends on whether there is sufficient river sediment supply, and the impact of relative sea level rise is relatively small. The river-mouth bar located in the active distributary mouth has maintained rapid growth in the past three decades, although the Three Gorges Dam has trapped most of the upstream sediment. We believe it is the sediment scoured from the main channel downstream of the dam substantially supplements the sediment discharged to the sea, contributing to the rapid growth of the river-mouth bar. Our finding inspires a reassessment of the effects of dam construction on delta evolution when sediment reserves downstream of the dam are large and the runoff is ample. Under the current circumstances of rapid sea level rise and intensified human activities, the rapid growth of river-mouth bars mainly due to the sediment supply from the river channels allows precious buffer time to protect the vulnerable Changjiang Delta, and it also brings hope for and new perspectives on the protection of other major deltas in the world.