Sediment transferring function of the lower reaches of the Yellow River influenced by drainage basin factors and human activities

Sediment transferring function (F_s) of rivers is defined and indexed in this study, based on the concept of sediment budget at river reach scales. Then, study is made on the F_s of the lower reaches of the Yellow River in relation to natural and human factors in the drainage basin, such as the annual precipitation in different water and sediment source areas, proportion of >0.05 mm sediment of the total sediment load to the lower reaches of Yellow River, the regulated degree of the “clear” baseflow from the drainage area above Lanzhou, frequency of hyperconcentrated flows, area of soil and water conservation measures in the drainage basin. As a result, a multiple regression equation has been established between F _s and 7 influencing factors, with multiple correlation coefficient r = 0.90. The reduction in annual precipitation in different water and sediment source areas has different effects on F _s. The reduction in annual precipitation in the area above Hekou Town and the area between Longmen and Sanmenxia results in a reduction in F _s, but the reduction in annual precipitation in the area between Hekou Town and Longmen results in an increase in F _s. The grain size composition of sediment load strongly affects the F _s; the larger the proportion of >0.05 mm sediment in the suspended sediment load entering the lower reaches of the Yellow River is, the lower the F _s will be. Thus, if the Xiaolangdi Reservoir traps coarse sediment and releases fine sediment, F _s will be enhanced. This study also shows that the lower the proportion of high-flow season river flow to the annual total river flow at Lanzhou Station is, the lower the F _s will be. Therefore, the interception of large quantities of clear water by the reservoirs on the upper reaches of Yellow River is one of the major causes for the decrease in F_s in the lower reaches of the Yellow River.