A horizontal movement of water, as along a stream or through an ocean. The rate of flow of a river current varies with depth because friction operates along the bed and sides (R. Chorley et al. 1985). Tidal currents are associated with the rise and fall of the sea, and the velocities of ebb and flow vary with the morphology of the coast and any outflow of fresh water; see FitzGerald et al. (2002) Geomorph. 48, 1–2. Rip currents are generally strong, shore-normal, jet-like flows that originate within the surf zone and are directed seaward through the breakers. They form in the nearshore zone and balance the inflow of seashore currents, influence the morphology of the shoreline, and may be important for transporting fine sediments offshore (MacMahan et al. (2005) Marine Geol. 214, 1–4). Ocean currents are driven by the planetary winds; see Hidaka (1952) J. Oceanograph. Soc. Japan (available online, captivating English, many equations).