It is well known that sediment sorting according to size, shape and density takes place but it is not known just how it takes place. To assess the effects of size and density sorting, sand-size spheres of different density were transported under controlled flume conditions. Observations on the motion of discrete grains show that grains smaller than the roughness elements move continuously and have the same velocities regardless of density. For grains near and slightly larger than the roughness, movement is intermittent and, for a given size, heavy particles move more slowly than lights. For grains much larger than the roughness, movement is once again continuous over the rough surface and light and heavy grains move at nearly the same velocity.

Bulk samples containing spheres of a range of sizes and two densities were also studied. Analyses of grain size for the two densities show that for planebed transport, the size of heavies decreased with respect to that of lights with distance transported. For ripple-bed transport however, the size relations between associated light and heavy grains remained essentially unchanged for the duration of transport. These results indicate that the specific process of sediment sorting is a function of the exact mechanism of sediment motion.