Gravity has the upper hand. Disintegrated material is moved to lower levels, leaving fresh bedrock exposed. This is the kind of sediment transport performed by rivers and some surface and deep-sea currents. Much sediment is transported in the medium of water, either in streams and rivers or by ocean currents. Once they are in motion, sediment particles can move in a variety of ways.
Grains slide or roll over the substrate.
Grains hop or bounce along the substrate. Varying conditions can cause the sediment to change from one type of movement to another. Grains orient themselves in order to present the least resistance to flow or to find the most stable position against surrounding grains. SIZE is also a factor, but it is not well understood. The flow strength must increase to the level where the bed load is mobilized.