Currents are formed by the motion of water. A river current is gravity taking the water downward, given direction by the topography of the river bed. In larger bodies of water--most notably oceans--currents are formed as part of complex cycles. Some currents form and deplete with little or no pattern, while others always flow, though the motion and direction may shift.
Technically, currents can occur anywhere there is water. The most powerful currents form in rivers and oceans, where they can have an effect on ecosystems and navigation. With larger bodies of water, currents have layers formed by wind, tides and gravity. The flow of rain and runoff can form currents. Some ocean currents flow continuously and are part of large, rotating loops called "gyres." Others form as the sudden result of storm activity. Current is a basic characteristic of what makes a body of water a river and form as water flows down a land mass towards the ocean.
The major ocean currents circulate and form in ongoing, rotating patterns. These form from the accumulating effect of gravity, the motion of the Earth and Moon, and surface weather. Currents like rip-tides are smaller, but often powerful, and form as the result of wave patterns and water circulating back out to sea. Wind currents are created as wind pushes water on the surface, with large ocean currents being formed by blowing winds such as the westerlies and the trade winds. River currents range form slow flowing to whitewater rapid.
Currents form differently because of the Earth's spinning rotation. The result is the Coriolis effect, which causes the water to spin clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
Currents also are formed as a result of warm water from the equator meeting the cold water of the poles, created as cold water flows under the warm. The water cycles form currents that help replenish oxygen in the ocean to create ecosystems that support life.
Currents that are formed suddenly or are not perceptible by looking at the surface of the water can be dangerous. Some whitewater currents may pull swimmers underwater and make it impossible for them to surface. Rip currents can be impossible to detect until swimmers realize they are being pulled out to sea. People engaging in water sports should be aware of the conditions that can cause these types of currents.
- Photo Credit ocean image by sergey2008 from Fotolia.com