Nearly all rivers eventually run into the sea. The region where a river runs into the ocean is called a river delta. The river delta is prone to frequent flood events, and is also strongly affected by the tides.
The coastal environment is known as the littoral zone. The littoral zone extends from the highest water line that occurs on shore during a storm, to the point in the ocean where the land is too deep for storm waves to move sediments on the floor (60 m or 200 feet). The point of contact between the ocean and the land is known as the shoreline or the coastline. Mean Sea Level (MSL) is a value based on the average tidal levels recorded hourly at a given site over many years.
The ocean is a powerful force in the shaping of the lithosphere. Ocean waves are continually weathering, or wearing away at the land and changing the shape of the coastline of our continents. Coastlines in areas of active tectonic uplift, such as along the western coast of North and South America, tend to be rugged and high relief. Sea cliffs are formed by the undercutting action of the sea. As the waves wear away, a greater amount of the land along the coast, gravity will often take over and cause a portion of the cliff to fall away into the sea.
While in areas of active tectonic uplift, the ocean is primarily an erosional force, in passive margins, such as along the eastern coast of North and South America, wave are a constructive force. Waves create small islands of sand parallel to the coast line, called barrier islands or barrier spits. If a barrier island grows so large that it connects with the land, it can form a lagoon.
The currents of ocean waves transport sediments (primarily sand) along the coastline in a process known as longshore drift. Beaches are not just made up of white sand. From the standpoint of physical geography, beaches are the portion of the coastline where accumulated sediment is in motion, so to geographers, beaches can be made up of gravel, rocks or boulders. Beaches act to stabilize a shoreline by absorbing wave energy.