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Settling velocity for a particle of sediment can be derived using Stokes Law assuming quiescent (or still) fluid in steady state. The resulting formulation for settling velocity is,

$w_{s}={\frac {g~({\frac {\rho _{s}-\rho }{\rho }})~d_{sed}^{2}}{18\nu }}$,

where $g$ is the gravitational constant, 9.81 $m/s^{2}$; $\rho _{s}$ is the density of the sediment; $\rho$ is the density of water; $d_{sed}$ is the sediment particle diameter (commonly assumed to be the median particle diameter, often referred to as $d_{50}$ in field studies); and $\nu$ is the molecular viscosity of water. The Stokes settling velocity can be thought of the terminal velocity resulting from balancing a particles' buoyant (proportional to the cross-sectional area) and gravitational forces (proportional to the mass). Small particles will have a slower settling velocity than heavier particles, as seen in the figure. This has implications for many aspects of sediment transport, for example, how far downstream a particle might be advected in a river.

A plot showing the relationship between sediment particle diameter and the Stokes settling velocity