Based on the understandings of the relationship between the river runoff and the primary functions of the Yellow River, this paper advances the concept of functional no-flow event that is different from the zero-discharge event occurring in the river. Factors reflecting the characteristics of functional no-flow events were analyzed and the criteria describing severity levels of the events were given from the cluster analysis with the measured data from 1950 to 1999. The results show that the severity level is subject to the critical water demand for river ecosystem protection. Using two critical water use demand scenarios for the Yellow River proposed by the authors and the Yellow River Water Conservancy Commission, different severity levels of functional no-flow events are identified. In addition, the differences between the zero-discharge and the functional no-flow events are discussed.