FUNCTION CHANGES OF THE CHANGJIANG RIVER IN THE QUATERNARY

Our study indicates that the functions of the Changjiang River have changed greatly during the Quaternary. The Jinsha River, located in the upper reaches of the Changjiang River, has cut across several rivers which southward flowed into fault basins, and eastward flowed into Sichuan Basin, formed the Ya'an Gravel Stratum in Sichuan Province in about 1000 ka BP. Then it has cut and formed the Three Gorges. Below Yichang it formed extensive gravel fan deposits. In the channel of the middle-lower reaches it usually developed sandy gravel deposit. Therefore, the functions of the Changjiang River in the Medio Pleistocene mainly were deeply cutting and transporting coarse gravels. In the Pleniglacial of the Epipleistocene, it developed coarse gravels and formed calcitic cementations in the upper reaches and deeply cut to develop many deep troughs in riverbed in the middle-lower reaches, which were at an elevation from -20m--60m. At the same time, the great amounts of sediment were transported to the continental shelf plain. In the Holocene, flood level had risen continuously, and developed fluvial deposits and floodplain extensively. In the zone along the middle-lower reaches, the thickness of the Holocene deposition reached 30-40m, which has gone far beyond the normal thickness of the alluvial flat. Since 2000 a BP, discharging flood and sediment into the sea was the main function of the River. It relieved flood overflowing and silt depositing on the flood plains along the river. But near the river mouth, the sediment depositing increased rapidly and it speeded up the river delta development. Causations for the function changes of the River are inferred in the paper as below: Along with the uplift of the Tibet Plateau, the Western Sichuan Plateau and the Yunnan Plateau had upheaved in succession, and resulted in the Jinsha River flowing eastward. Alternations of glacial and interglacial ages had resulted in the intensity changes of the deep cutting of the Changjiang River, and developed gravel fans in different periods. Sea level rise had resulted in frequent floods in the middle-lower reaches, and the wide floodplains were formed along the banks of the middle-lower reaches. After constructing dykes along the middle-lower reaches, it increased the flood and sediment discharge into the sea, which speeded up the river delta development during the last 2000 years.