In order to gain insight into the sediment transport process as it occurs in overland flow, velocities of fine to coarse sand grains were measured on beds of varying roughness. For all grain sizes, grain velocity can be well related to the shear velocity of the flow. However, the relationship is essentially linear for coarse grains, while there is a progressive increase of grain velocity for fine grains. The latter is not only due to the changing flow characteristics with height, but also to the fact that fine grains are transported at relatively greater heights above the bed at high shear velocities. The results explain some characteristics of earlier developed empirical transporting capacity relationships.