A beach is a strip of coastal land above the water that is covered at low tide and exposed at high tide. It is characterized by the presence of sand, cobbles and gravel (or other material). A beach may also contain marine organisms including shells, algae, crabs and sea anemones. Beaches are found around the world, especially in tropical climates.
A major function of beaches is the accumulation and deposition of sediments. The most common materials are sand, silt and gravel. These are derived from erosion of the land and delivered by rivers, glaciers, shoreline erosion, wind and waves. Beach sand is of a fine to medium size, while beach gravel consists of coarser rock fragments. Beach cobbles are usually the result of mechanical weathering of rocks by wave action.
The composition and structure of a beach depend on the source of the sediments. Generally, beaches with a more abundant supply of sediments tend to accrete or build up; conversely, those with a less-available supply will erode. Beach characteristics also vary with the size and direction of the prevailing winds, tidal movements and the obliqueness of the waves.
Beaches are not only found on the ocean shore, they are also common along lakes, rivers and some large ponds. Beaches can be characterized by their sandy or pebbled surface, the presence of seaweed and other marine organisms, the relative calmness of the water and the availability of public access.
All beaches are continually changing, with new materials being added and old materials removed. These processes are mainly driven by the action of waves and longshore currents. Beach sediments can include sand, silt, clay and rock fragments. Coarse grained materials, such as cobble and shingle, are often deposited by surf and tidal flows, while finer grained materials such as mud or silt, which form mud when wet, can be moved seaward by waves and tidal currents to create sand bars and sediment drift zones.
Beaches are also subject to pollution from sewage, industrial and agricultural waste, and human activities such as fishing, swimming, picnicking, sunbathing and recreational boating. These substances can cause illness, disfigurement and even death in humans and animals. Despite this, many beaches are considered beautiful places where people come to relax and enjoy the water. Many jurisdictions make all beaches public, while others have rules about where people can and cannot go on private beach land. Some beaches are closed after severe storms as a result of toxic contamination from raw sewage and other pollutants.