A beach is a place where land meets water. It is home to many plants and animals, including hermit crabs scurrying across the sand and fish swimming in the waves. Beaches are also often lined with seaweed, which provides shelter to shrimp and other small creatures. Beaches change over time as the tides wash in and out, erosion and deposition occur, and sand is moved around by surf and winds.
Some beaches are wide, others narrow, some steep and some gentle, some with a lot of shells and some without. They are all formed by different processes, and it is interesting to see how a beach evolves over time. A sandbar is a submerged ridge of sand that forms offshore from a beach. It is usually exposed at low tide, although it may disappear at higher tides. Sandbars form when the swirling turbulence of breaking waves excavates a trough in the sandy bottom. The sand is then carried by currents away from the beach and deposited elsewhere. This process is called longshore drift and it moves sand south along both coasts of the United States. Beaches are built and re-built as the drifting sand is swept inshore by the incoming waves. In some places, the sand is left on the beach as fingerlike features known as spits or baymouth bars. In other places, it is deposited in open bays as tombolo, or small islands, or in gulfs as barrier islands and marshes.
When the sand reaches the beach it is washed ashore by the waves, which then cover it with silt and clay. These fine-grained sediments are the weathering products of rocks found inland. When they are washed up on the beach they form mud layers, which can be seen after storms. Beaches with coarse-grained sand have less backwash and tend to build a smoother slope.
Waves can cause rips that scour the sandbars, making them erode and break apart. This is a good thing because it keeps the beaches from becoming too steep, but it can make them dangerous to swimmers. Beaches are also subject to pollution from runoff from urban areas, rivers and drainage pipes. Sometimes raw sewage and other toxic chemicals wash up on the beaches, causing health concerns. A lot of work is done to keep beaches clean and healthy, but it is not always successful. In some cases, beaches are closed after strong storms when the amount of bacteria and other pollutants washed up on the beach is too great. It takes days or even weeks for the waters to return to safe levels.