Alluvium is any material laid down by a river. The smaller pieces are also often called silt or mud. The larger pieces are called pebbles, cobbles and boulders.

All of these materials are carried by a river and so are all alluvium. However, the finer materials are the ones that are spread over the floodplain. The larger and heavier materials stay in the river channel.

Alluvium is laid down on the bed of a river and also across the surrounding land when a river floods over its banks. The material it is carrying then settles out and creates a layer. This layer is often silty or muddy and this is why people use the words silt or mud instead of alluvium. However, it may also contain sand and even pebbles.

Alluvium is the material which builds up to make the flat land either side of a river that is called a flood plain.

You can see the thickness of alluvium by looking at the banks of a river.

This picture shows you some of the many sizes of material that are carried by a river and that can be called alluvium. The smallest pieces that you can see individually here are sand. The fine material around the sand is silt and mud. The largest sizes shown here are pebbles.