Abstract: Semi arid climate, poor vegetation cover and highly erodible loess can be regarded as the main natural factors responsible for the formation of hyperconcentrated flows in the Loess Plateau, China. The degree, to which the hyperconcentrated flows develop, especially their extreme suspended concentration, shows a significant areal differentiation. This is closely related to the grain size composition of suspended sediment. To study the relation between hyperconcentrated flows and the surface material, the mean annual suspended concentration C mean and C max suspended concentration C max have been plotted against the >0 05 mm and <0 01 mm percentages ( R >0 05 mm, R <0 01 mm) in suspended load, respectively. For each curve obtained, there is a peak value. The peak value appears where R >0 05 mm is around 40% or R <0 01 mm is around 20%. This fact means that there exists an optimal grain size composition of suspended load, which makes suspended concentration reach the highest value. The formative mechanism for this optimal grainsize composition can be explained by the special physical and mechanical properties of hyperconcentrated flow as a solid liquid two phase flow. When the fine sediment is much more but the coarse particles are not sufficient, the flow may enter a laminar state; when the coarse particles are much more but the fine sediment is not sufficient, the coarse particles can not be set in suspension effectively. In these two cases, suspended concentration cannot reach very high. Obviously, between the two ends, an optimal grainsize composition exists at which the suspended concentration may be the highest. This optimal composition is related to the combination of three surface material types, i.e. loess, wind blown sand and weathered material of bedrock, and it occurs as a result of seasonally alternating wind and water processes operating in this area. It was found that in the middle Huanghe River basin, suspended sediment is coarser than that of the loess in the basin, but finer than the wind blown sand and bedrock weathered material. So, it is some proper combination of materials supplied from these sources that forms the optimal grainsize composition. During winter and spring, wind carries coarse aeolian sand to gullies and river channels. It has been observed in the field that, driven by strong wind, aeolian sand dunes on floodplains may approach the river bank and then collapses into the channel. This process is enhanced by bank erosion. When floods occur in summer, the aeolian sand previously supplied to the channel may be carried downstream by stream flow. During the floods that come from loess covered areas and lack of coarse sediment, the previously stored coarse aeolian sand may be set suspension by the fine sediment rich water flow and thereby its suspended concentration increases sharply, leading to the formation of hyperconcentrated flows.

Key words: hyperconcentrated flows, physico geographical factors, concentration of suspended sediment, optimal giansize composition, alternative wind and water processes, Loess Plateau