Individual grains move through gravel bed rivers in cycles of motion and
rest with variable characteristics, so tracer grains spread apart as
they transport downstream in a type of diffusion. Experiments and
Newtonian simulations have demonstrated nuanced diffusion
characteristics, with at least three distinct ranges of behavior as the
observation time of tracers increases and each range exhibiting a
different spreading rate. Although these observations are nearly 20
years old, no physical model has been developed to describe them,
leaving us uncertain of the generating processes. In this work, we
develop the first physical model describing three bedload diffusion
ranges by incorporating sediment motion, rest, and burial into a random
walk concept of individual bedload trajectories. Using the model, we
attribute multiple bedload diffusion ranges to the interplay between
motion, rest, and burial processes, and we relate the multi-range
diffusion characteristics to measurable transport parameters.