sediment (mud, sand, and gravel) laid down by flowing water. The largest particles (sand and gravel) tend to accumulate within the channel itself. Particles of clay, silt, and fine sand are small enough to be suspended in flowing water. When the stream overflows its banks, these particles can be distributed across the valley floor. These overbank or flood deposits are the most common contexts in which buried archaeological sites are found. Stream valley floors are underlain by deposits of alluvium and often contain buried archaeological sites..