Based on the relationship between relavent forces acted on particles and the angle of repose at the condition of incipient motion,two pickup probability models were proposed for sediment entrainment under the normally distributed instaneous velocity and log-normally distributed instantaneous velocity, then two incipient velocity for nonuniform sediment models were derived. The results were compared with the published experimental data and field data and the results showed that the mechanism of incipient motion of gravel and cobble which are the main components of mountain river bed may be different from that of sand. The analysis showed that the results for the log-normal distribution reveal a significant improvement of accuracy for the mixtures with or without gravel and cobble.
