Current

A sustained movement of water with a measurable direction and speed.

Currents may be driven by wind, density differences, large-scale ocean circulation, river flow, pressure gradients or other processes. Tidal streams are periodic horizontal water movements caused by the tide and are often treated separately in UK navigation terminology.

A current changes a vessel’s movement over the ground without necessarily changing its heading or speed through the water. Direction is commonly stated as the set towards which the water flows and speed as drift or rate, but conventions and reference frames should be confirmed.