A mechanistic-cum-stochastic theory describing the sediment transport phenomenon from the particle scale of entrainment to the continuum scale of bedload flux under a steady unidirectional flow over a compact sediment bed is developed. The sediment particles, considered as discrete spherical particles, are subjected to hydraulically smooth, transitional, and rough wall-shear flows. At the particle scale, the hydrodynamic forces acting on a sediment particle resting over three closely packed bed particles are analyzed. The criteria for entrainment threshold in rolling, sliding, and lifting modes are determined introducing the velocity fluctuations. Comparison of the computed threshold curves with the experimental data reveals that the entrainment threshold primarily belongs to the transition of the sliding to the lifting modes. Then, at the particle scale, the entrainment probabilities in rolling, sliding, and lifting modes for a given median size of sediment particles are obtained. The rolling and sliding probabilities increase with an increase in Shields parameter, and after attaining their individual peak values, they decrease, while the lifting probability increases. Finally, the bedload flux, in a continuum scale, is obtained using the lifting probability. © 2017 American Society of Civil Engineers.