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Deposits on High Floodplain and Flood Evolution of Huanghe River in Lanzhou

According to the investigation of the section on high floodplain of the Huanghe(Yellow) River and grain size analyses of 81 samples in Lanzhou,the paper researched the sediment characteristic of the section and the flood variation at that time.Results indicate that the flood sediment layers of the Huanghe River are clear in Lanzhou area,which reflects clearly the variation of the floods and the high resolution short-term precipitation,and is a good target indicating the changes of the flood and precipitation.The coarse grain of the sediment indicates the bigger scale flood and the more precipitation at that time,and the fine grain indicates the smaller scale flood and the less precipitation.The Matan section is divided into 15 phases,which indicates 15 bigger-scale flood stages,and the sediment are mainly composed of the coarse silt,the very fine sand and the fine sand.The content of the coarse silt is the most,the very fine sand and the fine sand are the second and the colloidal particle,the cosmid,the fine silt,the medium sand and the coarse sand are all little.The grain composition of the whole section has obvious change rule from above to bottom.Sufficiently considering the effect of landform heightened by sediment on grain composition,the order of the depths and scales of floods on the high floodplain of Matan section from big to small is the 10th12th11th13th15th14th3rd5th7th8th6th1st4th9th2nd.Reckoned by the historical documents,these 15 floods that sediment layers of Matan section reflect occurred since 170 a B.P.The flood sediment is not continuous,each of 15 floods generally indicates transitory rainy season of one year,the interval of two floods is generally about 10 years,and in the interval of 10 years there are no floods or small scales floods,indicating there are normal or less precipitation.According to the causes of modern floods of Lanzhou,it is concluded that the causes of 15 floods of Matan section is the annual precipitation's obvious increase and the dates of the most precipitation are same or near in Huanghe River drainage basin above Lanzhou and branch drainage such as the Yao River,the Datong River,the Daxia River and so on.