A beach is a stretch of sand or other sediment that lies along the edge of a body of water, such as an ocean or a lake. It can be a place for swimming, sunbathing, and playing games. Beaches also provide habitats for many types of animals, such as hermit crabs and sea anemones. Beaches are constantly changing as tides and wind bring in new materials and take away others.
Beaches are often composed of sand, but they may contain any type of granular material that moving water can carry, such as silt, gravel, cobbles, shingle, or shells. The size of the particles that make up a beach is important because they help determine its shape and how it interacts with the rest of the coastline. Beaches are most often found along the ocean, but they can be seen on lakes, bays, and even ponds.
Sand is an important part of beaches, but it can also be found in rivers and streams, and in marshes and dunes on the land. The shape of a beach depends on the size and location of the source of the sand, as well as the movement of the sediment by waves and currents. For example, a beach that is shaped by waves that break close to shore may have more finely-grained sand than a beach that is shaped by longshore drift, a process that moves sand in a constant direction.
Waves and currents move the sand that makes up a beach, sorting it according to its size and density. The smaller, lighter sand grains are moved more easily by waves and currents, so they tend to get deposited at the beach. The larger, more-dense sand grains are moved further out into deeper water or inland to form sedimentary deposits such as deltas.
The sand composition of a beach can also be affected by its environment, such as the presence of vegetation and the speed and turbidity of the water and wind. For example, dense, well-compacted sand tends to resist erosion from wind and waves. Vegetation also helps protect beaches by slowing the flow of sand and reducing erosion rates.
Beaches can be described in terms of their slope, the amount of beach sand available at high tide, and the number and location of sandbars. A bar is a submerged or partly exposed ridge of sand or other coarse sediment that was built by waves offshore from the beach. The location of sandbars on a beach can change seasonally because of the different types of wave energy experienced in summer and winter. During the winter, when waves are weaker, sandbars appear closer to the beach. When waves are stronger in the summer, they create sandbars further out into deep water. This creates a classic bar-trough system, with a series of sandbars separated by paddling channels.