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Upper- and lower-flow regimes are formed under different conditions. To identify them is important for predicting mobile bed resistance. Based on the dimensionless resistance equation, a new discriminant function to flow regimes is deduced, which contains two dimensionless parameters, namely, the suspension number and the flow action number. The concepts of the flow regime transition angle and the flow regime transition path are proposed. The problem of the discriminant flow regime is revisited from the perspective of flow resistance. The discriminant function is calibrated by 4 176 sets of flume and field measurement data at home and abroad, and the critical equation between the upper-and lower-flow regimes is determined. The results show that the prediction accuracy of the proposed critical equation is approximately 91% in the lower regime and 85% in the upper regime, and the comprehensive accuracy is approximately 89%. In fine-sand rivers, when the water temperature decreases, the transition from the lower-flow regime to the upper-flow regime is more likely to occur. The discriminant function proposed in this study can not only identify the flow regime with high accuracy but also analyze the effect of water temperature on the flow regime transition. [ABSTRACT FROM AUTHOR]