Jetty is a pier or wall built out from the shore of a river, lake, or ocean to control the flow of water. A jetty at a river mouth or harbor entrance may deepen a channel leading into the river or harbor so that ships will not run aground. A jetty also may extend a channel in order to prevent sand from blocking the channel’s mouth. A type of jetty called a groin protects shores from erosion by trapping sand and silt. Jetties may be made of timber, stone, concrete, or steel, sometimes in combination with sand.
A jetty deepens a channel by narrowing it, causing the water flowing out of the channel to move faster. The faster-moving water picks up more particles of sand or mud from the bottom of the channel, thus deepening it. In areas where currents deposit sand around a channel’s mouth, a jetty can extend the length of the channel to make the current carry sand away from the mouth.
The Columbia River, which empties into the Pacific Ocean, has one of the world’s longest jetties. Its south jetty is about 61/2 miles (10.5 kilometers) long. The harbor has a mechanically dredged channel 55 to 60 feet (17 to 18 meters) deep.
