The name given to material that has been carried by rivers and then deposited. Sediment may also be called alluvium.

This picture shows cobbles on the edge of a river. These are carried by the river in times of flood - just as sand, silt and clay are - and so are part of the river sediment.

This picture shows a river carrying so much sediment in suspension that it has turned light grey. A tributary enters carrying little sediment, which is a clearer dark blue. The waters only mix much further downstream.

Sediment can be large, in which case it is called boulders, cobbles and pebbles, or it may be of medium size - gravel and sand - or it may be fine - silt - or very fine - clay, or mud.

The finest materials are carried in suspension and do not easily settle out on the bottom of the river. The medium materials only move during times when the river is full of water and then they mostly hop along the river bed. The largest sediment moves only in times of flood and then it rolls along the river bed.