Riverbed scour of the main channel by floods in the Yellow River and its tributaries was investigated, including scour by hyper-concentrated floods. Flood scour usually causes variation of river cross-sections in a way similar to that occured when the sediment inflow is less than the sediment-laden capacity. Scour rate equation for the main channel derived based on the momentum and continuous equations was verified by field data. This equation indicates that unsteady flow scour rate is proportional to the flow density, the velocity of the flood peak, the rising rate of flow discharge per unit width, and so on. The Maximum scour depth after a flood could be predicted by the scour rate equation proposed in this paper. Riverbed scour of the main channel by floods in the Yellow River and its tributaries was investigated, including scour by hyper-concentrated floods. Flood scour usually causes variation of river cross-sections in a similar to that occured when the sediment inflow is less than than the sediment-laden capacity. Scour rate equation for the main channel derived based on the momentum and continuous equations was verified by field data. This equation shows that unsteady flow scour rate is proportional to the flow density, the velocity of the flood peak, the rising rate of flow discharge per unit width, and so on. The Maximum scour depth after a flood could be predicted by the scour rate equation proposed in this paper.