The intrusion of a denser fluid beneath a lighter fluid, due mainly to the hydrostatic forces arising from gravity and the density differences. This term is used principally in engineering for such cases as the intrusion of saltwater below freshwater in an estuary, or for currents caused by the presence of denser water with suspended silt at the bottom of a lake or ocean. Many of the phenomena are quite analogous to some of those associated with cold fronts in the atmosphere.