Changes Of Sediment Load From The Main Tributaries In The Middle Reaches Of The Yellow River And Their Impacts On The Sediment Flux To The Sea

The sediment budget of Huanghe (Yellow River) Delta has been severely disturbed, as a result of the sharply reduction in sediment and water delivery to the delta due to human activities in the drainage basin. These have triggered a series of environmental problems, such as the shrinking of delta, shoreline erosion, sea water intrusion and deteriorated coastal ecosystem. The Huanghe sediment delivery to delta is not only the critical material basis for constructing the Delta, but also the key precondition to maintain the delta stability. The middle reaches of Huanghe are the major sediment yield areas of the Huanghe Basin, and thus their changes in sediment yield play a direct and dominant role in the evolution of Huanghe Delta. Therefore, a comprehensive study of changes in the main tributaries'sediment yield and its controlling factors, would provide important scientific and practical insights to 1) the variation of sediment delivered from the Huanghe to the sea, 2) the evolution of the Huanghe Delta, and 3) the drainage basin-coastal zone interaction of large river in Anthropocene.This study firstly analyzes the variations of annual water discharge and sediment load of seven main tributaries in the middle reaches of Huanghe, based on the annual data of water and sediment at the seven main hydrographic stations of tributaries (Huangfuchuan River, Kuyehe River, Wudinghe River, Yanhe River, Beiluohe River, Jinghe River and Weihe River) and Tonguan Station in the middle reaches of Huanghe in 1950-2009. The results show that the annual water discharge and sediment load decreased sharply over the past 60 years, the total sediment loads have decreased from 13.5×10~8t/a in 1950s-1960s to only 3.4×10~8t/a in 2000-2009, with a reduction of about 75%, which is attributed to regional climate change (e.g. decreasing rainfall), in combination with intensive human activities (including soil and water conservation, agricultural and industrial water consume, and the return of farmland to forests, etc.). The results of sediment rating curves indicate that, the relationship (R~2) between water discharge and sediment load in the four north tributaries are more significant than that of the other three south tributaries because of the spatial difference of intensity of human disturbance(e.g. soil conservation, vegetation cover, etc.).Secondly, the seasonal changes in the seven tributaries are discussed, using the monthly data of water and sediment in 1950-1989. Results show that these rivers are under condition of slight flooding in March-April and major flooding in July-September. Annually, the sediment transport mainly occurres in July-August. The seasonal sediment rating curves show that, even a small change in water discharge will cause larger change in the sediment loads during the flood seasons than that in dry seasons, presenting a magnifying effect of flooding water discharge on sediment delivery.We analyze annual sediment load of the seven major tributaries, in combination with the data from several hydrological stations in the mainstream (e.g. Tongguan, Huayuankou, and Lijin). Results show that, the seven tributaries played a controlling role in the decreasing Huanghe sediment flux to the sea, while the rapid increases in water consumption and dam-orientated regulation and entrapment in the low reaches since the 1980s, enhanced the reduction of sediment flux to the sea.Strongly impacted by climate change and human activities, the sharp decrease in sediment delivery from the major tributaries in combination with the impacts of dams and water abstraction/consumption have caused rapid decrease of the Huanghe sediment flux to the sea over the past 60 years, which induces profound responses in the environment of the Huanghe delta. This work presents important significance to the drainage basin-coastal interactions of large river under global change.