A review of paleofloods in the middle-lower reaches of the Yangtze River during the Holocene: Processes, causes and effects

The understanding of the flooding processes in the entire basin of the middle-lower reaches of the Yangtze River (MLRYR) during the Holocene remains elusive. This is primarily attributed to the constraints posed by site-scale data characterized by limited spatiotemporal resolutions, compounded by conflicting results of the reconstructed Holocene paleoflood records in some of the previous studies. In this study, by synthesizing 114 paleoflood data with robust evidence of flood occurrence (including timing and location of flood occurrence), we comprehensively reconstructed the first continuous Holocene paleoflood record that cover the entire basin of the MLRYR. The results show that at the onset of the Holocene, flood frequency peaked notably in the middle reaches of the Yangtze River (MRYR), with no corresponding peak observed in the lower reaches of the Yangtze River (LRYR). However, in both the MRYR and LRYR, a significant flood frequency peak emerged around 8.0 +/- 0.5 ka BP, and scarce flood occurrences appeared approximately 7.5-5.0 ka BP. This scarcity shifted abruptly to a surge in flood frequency from 5.0-4.0 ka BP. Following this, the MRYR witnessed three successive peaks in flood frequency, occurring at approximately 3.0 ka BP, 1.8 ka BP, and 1.0 ka BP, respectively. In contrast, flood events in the LRYR were infrequent during the 3.0-2.0 ka BP period, followed by a surge in frequency from 2.0 ka BP onwards. We further explored the driving mechanisms of paleofloods in the MLRYR and found that floods were more likely to occur during periods of weakened EASM, characterized by wet and unstable climate conditions in the MLRYR. Marine erosion may be also a key factor in the lack of geological evidence for palaeofloods during the early Holocene. Additionally, variations in the ISM and human activities during the late Holocene have significantly influenced the occurrence of floods in the MLRYR. By comparing the paleoflood frequency with the spatial distribution and number of archaeological sites in the MLRYR, respectively, we propose that the period of flood scarcity during the middle Holocene may have facilitated the development of rice agriculture and the prosperity of ancient settlements. In contrast, during flood-prone period, ancient societies adapted and coped with floods by migrating to higher terrain during the early Holocene and implementing simple hydraulic engineering techniques during the late Holocene.