Abstract: Focus on "what concentration sediment can be flowed away by different flow", "how many sediment can be scoured by different flow" these two question, the threshold of water and sediment in desert wide valley reaches upper the Yellow River are further studied in this paper. By the annual water and sediment process of each section, the scouring and silting law of each section in desert wide valley reaches are revealed to obtain threshold of water and sediment in different intervals under different concentrations. Based on principle of water and sediment balance, the relationship among flow, sediment discharge and sediment concentration are analyzed to establish sediment discharge model of flood. Then the transformation effects of water and sediment are quantized clearly. A case study shows that the sediment of Toudaoguai is 12.4 million tons and the sediment erosion is 5.27 million tons without considering optimization of water and sediment,while the sediment of Toudaoguai increased by nearly 4 times with considering optimization of water and sediment. The water and sediment regulation effect significantly improved the relationship between water and sediment, the desert valley river channel scour; verify the accuracy and reliability calculation of water and sediment and sediment discharge threshold. The research has important significance on efforts and scheme optimization of water and sediment in desert wide valley reaches upper the Yellow River.

Key words: threshold of water and sediment, sediment discharge model, sediment concentration, flow, scouring and silting law