A geomorphological process, a special case of the transportaton of sediment, involving particles that bounce along the bottom surface of the transporting fluid (a river, or the wind).

In particle sorting I discuss how the velocity of a moving fluid carrying sediment determines the size of the particles the fluid can carry. At a given velocity, the fluid can carry particles below a certain threshold size, but cannot carry particles above that size.

For particles right near the threshold, this situation is metastable -- minor variations in the current, and the orientation of a particle, carry it back and forth across the threshold repeatedly. In other words, the fluid picks up and drops the particle over and over; the particle will appear or bounce along the bottom surface of the current (the bottom of the river, or the ground).

Bouncing particles can cause small particles to break loose from themselves, or from the bottom. Saltation, then, contributes to the erosion of the surface in question.