In fluid dynamics, a gravity current is a primarily horizontal flow in a gravitational field that is driven by a density difference, hence gravity currents also sometimes being referred to as "density currents". When a gravity current propagates along a plane of neutral buoyancy within a stratified ambient fluid, it is known as a Gravity Current Intrusion. Typically, the density difference is small enough for the Boussinesq approximation to be valid. Gravity currents can be either finite in volume, such as the release from a dam break, or continuously supplied from a source, such as in doorway or lava flows. Gravity currents are typically of very low aspect ratio: they are much longer than they are high. It can be shown using dimensional analysis that typical vertical velocities are much smaller than typical horizontal velocities in the current, and so the pressure distribution is thus approximately hydrostatic, apart from near the leading edge. Thus gravity currents may be simulated by the shallow water equations, with special dispensation for the leading edge which behaves as a discontinuity.