Most of us know that tides are driven by the gravitational forces exerted upon the Earth by the sun and the moon. Tides are characterized by water on the planet being pulled by these gravitational forces, thus increasing and decreasing water depth systematically and periodically over time.
The motion of water that runs from low water to high water and vice versa is tidal. That tidal stream is sometimes referred to as a tidal current. Tidal currents are located close to shore and in bays, inlets and estuaries. They are different to other currents in that they are generated by tidal flows and are, therefore periodic and predictable.
There are additional currents generated by other factors such as wind or thermohaline circulation (a convection driven by salinity and temperature of the ocean). Thermohaline currents are found at all depths of the oceans.
Ocean currents are harder to predict although pilot books have been compiled over time and the approximate position, direction and speed of ocean currents can be estimated dependent on the time of year.
In some locations, such as the Strait of Gibraltar, we have tables which show us tidal streams and the surface current flowing into the Mediterranean from the Atlantic. This is an excellent example of different currents.
There is a surface current, which flows eastward through the centre of the channel, except when affected by easterly winds. This surface movement exceeds a westward flow of heavier, colder, and more saline water, which takes place below a depth of about 400 feet (120 metres). There is also a tidal stream which changes every six hours.