Beaches are normally composed of sand which comes from weathered rock via rivers. The sand is deposited by rivers when they reach the ocean, so that beaches are wide near river mouths. The littoral current transports sand away from the river mouths and along the shore. In some places, there is no sand available to build beaches. In those cases, the beaches may be composed of gravel or cobbles, volcanic ash or pumice, or even sea shells.
Seasonal variations can also occur in sediment supply.
Sediment which supplies the beach environment arrives in part due to longshore currents and littoral drift. The sediment may come from erosion of the beach upcurrent, but much of the sediment comes from river sediment discharge on the coastal margin. This sediment may have a strong seasonality depending on local climate. There can also be longer term variations in sediment flux and river discharge due to shifting climate patterns or changes in river channel. Because of variations in sediment supply and incoming wave energy, the process of beach erosion and deposition is continuous.