A beach is a strip of sand or other sediment that borders a body of water. It is made of small rocks (mainly sand and gravel) that have washed up from the ocean floor or other sources near the water, such as erosion of rock and coral reefs, or may contain the skeletons of molluscs. The shape of a beach depends on how waves move the sand. Waves that move up the beach and then down again create gentle slopes; those that break close to the shore cause steeper ones. A beach can also have features such as a reef, trough, or lagoon.
The beach is a natural habitat for many organisms, including crabs, insects, birds, sea turtles, and plants. Beach vegetation includes grasses, sea oats, dune flowers, and beach palms. Some beach plants have a network of roots that help stabilize dunes, and these are known as foredune plants. Beach flora also traps rainwater and enriches the surface layer of the dune, allowing other plant species to grow.
Many beaches have an area that is mostly above water during most of the day, called the beach berm. This is usually covered by a barrier of rocks, and it consists of the sand or other material that has washed up on the shore and then moved by waves. The beach berm has a crest and a face, with the face being the part of the dune that is nearest the water. It may also have a series of troughs or rip current channels that are gouged out of the beach by breaking waves, and it has large crescent elements, known as beach cusps, that are concave seaward.
There is a constant movement of sand on the beach, from north to south. This is because most waves come from the North Pacific on the west coast of the United States and the North Atlantic on the east.
Beaches are shaped by three types of forces: attrition, when large pieces of sand rub together; abrasion, when smaller pieces of sand scrape or dig into each other; and hydraulic action, where the motion of water is driven into cracks. Various factors affect these processes, such as the size and frequency of waves, the amount of wind, the amount of sunlight, the temperature of the water, and the type and amount of sediment in the sea. These influences can cause a beach to change significantly in only hours. Freak waves or storm surges can also substantially alter the location, shape, and appearance of a beach within minutes.