Tidal currents, as their name suggests, are generated by tides. Tides are essentially long, slow waves created by the gravitational pull of the moon, and to a lesser degree, the sun, on the earth's surface. Since the moon is so much closer to the earth than the sun, its pull has more influence on the tides.
The moon's gravitational pull forces the ocean to bulge outwards on opposite sides of the earth, which causes a rise in the water level in places that are aligned with the moon and a decrease in water levels halfway between those two places. This rise in water level is accompanied by a horizontal movement of water called the tidal current.
Tidal currents differ from the currents previously mentioned in that they don't quite flow as a continuous stream. They also switch directions every time the tide transitions between high and low. Although tides and tidal currents don't have much impact in the open oceans, they can create a rapid current of up to 15.5 miles (25 kilometers) per hour when they flow in and out of narrower areas like bays, estuaries and harbors [source: Skinner]. Fast tidal currents toss sediment around and affect plant and animal life. Currents may, for example, transfer a fish's eggs from an estuary out into the open sea or carry nutrients that the fish needs from the sea into the estuary.
The strongest tidal currents occur at or around the peak of high and low tides. When the tide is rising and the flow of the current is directed towards the shore, the tidal current is called the flood current, and when the tide is receding and the current is directed back out to sea, it is called the ebb current. Because the relative positions of the moon, sun and earth change at a known rate, tidal currents are predictable.
Currents, whether tidal, surface or deep ocean, profoundly affect the world as we know it. To learn more about the complex systems that drive ocean currents, dive into the links on the next page.