Depositional characteristics of sediments in the Changjiang River subaqueous delta during the catastrophic flood in 2020

Catastrophic floods in large rivers exert significant influences on the sedimentary environment in the estuarine region. However, viewpoints of sedimentary characteristics of the paleoflood in the estuarine delta is controversial. Therefore, it is necessary to understand the sedimentary characteristics of modern flood. The whole Changjiang River basin suffered from a catastrophic flood in 2020. Sixteen sediment cores in the Changjiang subaqueous delta were collected during the flood period. Grain size and organic index (TOC, TN) were measured. The result indicates that the flood layers is 3～21 cm thick, the average TOC and TN is 0.59% and 0.077%, respectively, which is higher than that of lower part deposit. The mean grain size of the flood layers (13.23 µm) is finer than that of the lower part deposits (13.87 µm). The end-member modelling analysis indicated that the finest populations (EM1) were originated from the 2020 flooding. Comparing the EMs of 2020 flood deposits with grain size characteristics of flood deposits in previous studies, we found that the 2020 flood sediments were finer than the lower part non-flood deposits, but coarser than the previous flood sediments, which is different from the common view that the paleoflood deposits are characterized by coarser sediments. We believed that this difference was due to (1) human activities (e.g. dam construction) in the river basin, by which more coarse sediment were trapped within the dam; (2) the weakened erosion in middle and lower reaches of river channels; and (3) the subaqueous delta by ocean dynamic erosion. This study is beneficial for the reconstruction of long-term paleoflood events sequences and the interpretation of extreme event deposition in the estuarine region.