Alluvium is a type of sediment or deposit that is formed by the accumulation of small particles that have been transported by water. It is usually found in riverbeds, floodplains, and other areas where water has been present for a long period of time. Alluvium can also be found in coastal areas, where it is deposited by waves and tides. Alluvium is composed of a variety of materials, including clay, silt, sand, gravel, and organic matter.