Nanjing Since The Last Glacial Maximum Characteristics And Evolution Of The Yangtze River

The Yangtze River from Nanjing to its mouth is stable braiding river. It has more than 400km span, and belongs to lower reaches and estuary. With the effect of climatic change and sea level fluctuating, the channel has sway many times from south to north or from north to south since the last glaciation maximum, but its general deflection is to south. Hundreds of boreholes are collected from the reach in Nanjing, Zhenjiang and Yangzhou, Jiangyin and Jingjiang, Nantong, Qidong. Based on these boreholes, the characteristic and times of sediment are emphasized to analyzed in Nanjing, and types and mechanism of channel migration from Nanjing to its estuary since last glaciations have been analyzed in this thesis, several results obtained:(1) There is three aggradation stages at least on the riverbed near Nanjing from last glaciation maximum, and have four types riverbed.Ⅰ stage is formed in last glaciation maximum and located in incised valley. The grain of sediment is coarse. The riverbed is narrow, runoff is small, but both drop of water and current velocity are large, motive power of the river is strong.Ⅱ stage is formed from late glaciation to early Holocene . The grain of sediment is coarse, too. Riverbed is wide, but drop of water and current velocity are large, motive power of river is strong. Though the temperature and sea level don't reach maximum, the precipitation of drainage area, runoff and sand transportation increases obviously.Ⅲ stage is formed in middle Holocene, the estuary is near Zhenjing and Yangzhou for a long time. Drop of water is smaller, riverbed is very broad, and runoff is very large, current velocity is small, aggradation is prominent. The width of the riverbed is 4-5 times than present riverbed near -20_-30m depth. In that time, high sea level ,high temperature and abundant precipitation exist in the same period, so the riverbed is very wide.The period of present riverbed formed. In late Holocene, climate is dryer and cooler than that in middle Holocene, the precipitation of drainage area decreases. So runoff deceases, the width of riverbed becomes narrow. The sea level stops in a high position for a long time, so both drop of water and current velocity are small.(2)There are two types of the channel migration since last glaciation maximum.The channel from Nanjing to Zhenjiang almost no change from last glaciation maximum now on, only the bed has sway at little range; The channel from Zhenjiang to estuary deflects to south in general. But the channel has sway many times from south to north or from north to south at large range.(3) The channel's change from Zhenjiang to estuary is effected by climate-sea level variation and Coriolis effect. In general, the channel moves to north in the period of high sea level, and moves to south in the period of lower sea level. The effect of Coriolis effect is obvious by making the estuary deflect to south constantly.