The oceans are constantly, restlessly moving. This movement takes place in the water column (the water in the ocean exclusive of the sea bed or other landforms) in the form of tides, waves, and currents, all of which affect the shoreline. (For a more complete discussion of the ocean's water column, see the chapter titled "Ocean.")

Tides

Tides are rhythmic movements of the oceans that cause a change in the surface level of the water. When the water level rises, it is called high tide. When it falls, it is called low tide. Along some shorelines, the tides are barely measurable. In other areas, however, the difference between high and low tide may be at least several feet (meters). High and low tides occur in a particular place at least once during each period of 24 hours and 51 minutes.

Tides are caused by a combination...