Abstract: Palaeoflood slackwater deposit is a major evidence used for inference about hydrological parameters of the past flood events. This fine-grained sediment is deposited at high flood stage from suspended load in overbank floodwater. Palaeoflood hydrological investigations were carried out at the Matouguan Reach of the middle Yellow River, Yonghe County, Shanxi Province, China. Four bedsets of palaeoflood slackwater deposits of the Holocene and palaeoflood indicators were found in backwater at FTC profile of the river. Palaeoflood SWD was identified by a variety of scientific sedimentological criteria such as color and structure obtained during field investigations. And the characteristics and hydrology parameters of sediment profile were also observed in the field work in order to provide data for hydrological recovery. Then these sedimental samples were analyzed on characteristics including particle-size distribution, magnetic susceptibility and loss on ignition. In conclusion, these samples were topical Holocene palaeoflood slackwater deposits in high water level stagnation environment. The peak stage of the sediment was recovered by the thickness and elevation, which were 527.34 m, 526.35 m, 521.28 m and 520.59 m. Sixteen cross-sections were selected at each site in one kilometer along the river reach. The estimated results obtained with HEC-RAS model indicate that the palaeoflood peak discharges were between 26 050~51 600 m³·s^-1. The validation results of the relationship between the ancient flood peak discharge and the basin area show that the results simulated in this paper are reasonable, indicating that the peak discharge of the Holocene palaeoflood in the Matouguan Reach of the middle Yellow River which was simulated by the HEC-RAS is scientific and reasonable. The sensitivity test of the model shows an error between -11%-9.40% with the same hydrological parameters, indicating that HEC-RAS model is more reliable than the slope-area methods. These four bedsets slackwater deposits recorded the extraordinary floods happened between 1 900-1 700 a BP and 3 400-3 000 a BP according to the results obtained by using Optically Stimulated Luminescence (OSL)dating method. The change of Holocene climate shows that the drought and flood disasters occurred frequently in these two period. The four floods in FTC reflects abnormal climate change during the two period and it is also the evidence of the changeful climate in Holocene. Results show that the Yellow River hydrologic system is in response to climate change, this confirms the rule how river in monsoon region responses to climate change, and provides more reliable evidence for the response between hydrological system and climate change. At the same time, flow simulation also provides the methods and data base for controlling the flood and mitigation in Yellow River.

Key words: holocene palaeoflood, HEC-RAS hydraulic model, hydrology, middle reach of the Yellow River