Sediment
Sediment is the type of material found on the bottom of a river or stream. It ranges from very fine silt through large boulders (silt - sand - gravel - stones - boulders). You may have any combination of types of sediment, however it is usually determined by the speed of the water. The type of sediments found in different places could affect what types of animals live in the stream.
Silt - Very fine particles; feels "squishy"
Sand - Slightly larger particles; feels gritty
Gravel - Up to size of a U.S. quarter
Stones - Up to size of a baseball
Boulder - Anything larger than a stone