Palaeoflood Sedimentological and Hydrological Studies on the Yongheguan Reach in the Middle Yellow River

Typical palaeoflood slackwater deposits of the Holocene age were found at many sites on both sides of the Shanxi-Shaanxi Gorges.A set of 5 beds palaeoflood SWD at the Yongheguang(YHG) site was sampled and investigated in detail.Analyses of the grain-size distribution,loss-on-ignition and CaCO3 content indicate that these SWD consist of sandy silt and silt fine sand.They were sourced from the suspended sediment load of the Yellow River floodwater,which was brought in by storm rains and floods from the triangle zone between Shanxi,Shaanxi and Inner Mongolia.They have recorded 5 extraordinary palaeoflood events on the Yellow River.The age of the palaeoflood events was determined by OSL dating of the sediment and stratigraphic correlation with the Holocene chronological framework established in the drainage basin of the middle reaches of the Yellow River.This shows that these extreme floods occurred during 3200-3000 a BP at the turn from the middle Holocene climatic optimum to the late Holocene dry period,which is the time of climatic decline from the Shang Dynasty to Western Zhou Dynasty,with occurrence of serious drought and flood disasters,harvest failure,famine and major social changes.The reconstructed peak discharges of the palaeoflood range from 28380 to 48590 m3/s,which are much larger than the gauged largest floods in the Shanxi-Shaanxi Gorges in the middle reaches of the Yellow River,thus the flood data series of the river extended to a 10000-year time-scale.