A channel-averaged deterministic approach is commonly used to study bedload transport in gravel-bed streams. Here an alternative formulation based on tracking the vertical and streamwise displacement of individual bed particles, and mass continuity of the sediment in the bed and bedload expressed in probabilistic terms is presented. Flume physical modeling of bedload transport with tracer stones is used for the development of the new theory. All experiments presented here were conducted under normal flow equilibrium transport conditions. The results obtained have been used to derive predictors for the probability density functions of bed elevation fluctuations, particle entrainment into bedload transport, and streamwise displacement of particles from entrainment to deposition.The formulation links tracer conservation with conservation of gravel as awhole, and in addition establishes a connection between vertical and streamwise dispersion of tracers with the overall bedload transport.