Abstract

By analyzing the sediment cores and cross-sections, this paper discusses the channel evolution of Yangtze River from Nanjing to the estuary since the global Last Glacial Maximum (LGM). During LGM the fall of global sea level led to intensified headward erosion, resulting the formation of deeply incised valleys. The postglacial transgression led to backfilling of the incised valley. From 7.5 to 6 ka B.P., a broad estuary bay and estuarine-shallow marine face were formed in the lower reach up to Zhenjiang. Since 6 ka B.P., because the sedimentation rate surpassed that of sea level rising, estuary bay was further filled up and subsequently the delta progradated, then the river channel narrowed and the river flew east-southeast. Gradually the present channel pattern came into being. Taking the cross-section of Nanjing reach as an example, the flow velocity and discharge of Yangtze River in various stages are estimated. The runoff volume are approximately 2517~2853m³/s, 27824~31570m³/s, 58265~66126m³/s respectively, according to the average velocity figured out above. Conclusions can be made that since the LGM the infilling of incised-valley accelerated due to the combined action of an increased discharge which carried more sediments and a slower flow rate due to the sea level rising. The sea-level changes, sea and land interaction is the main dynamic of the evolution of lower reach channel of the Yangtze River.

Keywords: Last Glacial Maximum, The Yangtze River from Nanjing, Incised-valley, The sea-level change