Abstract: Hyperconcentrated flow is a hazardous streamflow, which is typical in the Loess Plateau of China. Strong human activities such as deforestation and steep-land cultivation in this area can be traced back to 2000 years ago. Since the 1960s, large-scale soil conservation measures have been put into practice. All these have far-reaching effects on the hyperconcentrated flows. The middle Yellow River mainstream, the Wudinghe River and the Weihe River have been selected for a study on the influence of human activities on the hyperconcentrated flows. To quantify the occurrence of hyperconcentrated flows, the concept of the frequency of hyperconcentrated flows is proposed, which is defined as the number of days with daily suspended sediment concentration >300 kg/m³ in a given period, usually in a year. Then, this index is correlated with those indices that quantified the human activities. The results show that the frequency of hyperconcentrated flows is negatively correlated with forest cover, implying that the deforestation by humans in history may lead to a higher frequency of hyperconcentrated flows. The data from the Wudinghe River and the Weihe River show that the frequency of hyperconcentrated flows declines with the increase of the area with soil control measures and with the sediment reduction by soil conservation measures. However, the interception of clear water from the upper Yellow River by the Longyangxia and Liujiaxia reservoirs significantly increases the frequency of hyperconcentrated flows in the middle Yellow River mainstream. The temporal variation of hyperconcentrated flows in the past 50 years can be generalized as a three-stage model. In the 1950s to 1960s, the frequency of hyperconcentrated flows increased due to the destruction of natural vegetation by humans. From the late 1960s to the mid-1980s, the practice of large-scale soil conservation measures resulted in a decline in the frequency of hyperconcentrated flows. However, the frequency of hyperconcentrated flows has increased again since the late 1980s, because large quantitis of clear water are intercepted by reservoirs in the upper Yellow River for hydro-power generation.