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In fluid dynamics, a gravity current is a primarily horizontal flow in a gravitational field that is driven by a density difference. Typically, the density difference is small enough for the Boussinesq approximation to be valid.
Gravity currents are typically of very low aspect ratio (that is, height over typical horizontal lengthscale). The pressure distribution is thus approximately hydrostatic, apart from near the leading edge (this may be seen using dimensional analysis). Thus gravity currents may be simulated by the shallow water equations, with special dispensation for the leading edge which behaves as a discontinuity.
The leading edge of a gravity current is a region in which relatively large volumes of ambient fluid are displaced. Mixing is intense and head is lost. According to one paradigm, the leading edge of a gravity current 'controls' the flow behind it: it provides a boundary condition for the flow.
The leading edge moves at a Froude number of about unity; estimates of the exact value vary between about 0.7 and 1.4.
Gravity currents are capable of transporting material across large horizontal distances. For example, turbidity currents on the seafloor may carry material thousands of kilometres.
Gravity currents occur at a variety of scales throughout nature. Examples include oceanic fronts, avalanches, seafloor turbidity currents, lahars, and lava flows.
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