晚更新世晚期以来的长江上游古洪水记录

THE PALEOFLOODING RECORD ALONG THE UP REACHES OF THE CHANGJIANG RIVER SINCE THE LATE PLEISTOCENE EPOCH

长江上游三峡河段主要的古洪水记录有 :1)三峡深槽的蚀积变化 ;2 )长江阶地粗粒沉积 ;3)长江的泛滥沉积 ;4 )长江的古洪水平流沉积。不同时间跨度不同类型古洪水记录的精度有较大的差别。古洪水记录显示 ,晚更新世晚期的 4 0～ 30kaB .P .,长江上游大洪水比 30kaB .P .以来的长江上游大洪水大得多 ;全新世以来 ,以 3983aB .P .前后的大洪水为相对最大 ;公元 1870年大洪水为 30 0 0aB .P .以来最大洪水 ;近百年来的实测洪水以公元 1981年洪水为最大。

Based on the analysis of geological and geomorphological characteristics of palaeoflood, the regime of the paleoflood in the upper valley of the Changjiang River since the Late Pleistocene Epoch has been outlined in the paper. The geological and geomorphological indicators of the paleoflood in the Three Gorges have been obtained mainly from:1)the changes of erosion deposition in the deep trough;2)gravel sediments on the terraces of the Changjiang River; 3) floodplain deposition along the river bank including sand and finer materials; and 4) sediments resulted from spilling of water of paleoflood. Abnormal events of paleoflood have been reconstructed through the studies of flooding sediments. Gravels found in the first terrace showing abnormal magnitude of the paleofloods have been dated back to the Late Pleistocene period, 40～30kaB.P. which was more than 23.8% of those at the present in the upper valley of the Changjiang River. The extreme flooding events also resulted in strong erosion of deeper troughs. The earliest record of paleoflood during the Holocene period can be dated back to 7 810±445aB.P. The largest flooding events recorded in the upper valley of the Changjiang River occurred separately during 4 840～3 983aB.P. and 3 983±483aB.P.The abnormal flooding event recorded newly occurred in 1870A.D.being the largest since 3 000 years according to paleofloods investigation. The flooding in 1981 was the major events since 100 years.