The finest size of material carried in a river is called clay. It's also often called mud. It is the material that quickly colours the water during times of high flow and flood. Mud is left behind after a flood, coating the floors and walls of flooded buildings.

Clay and mud are also the most fertile part of the material carried by a river, so that when mud is laid down over a floodplain during a flood, it brings natural fertility to the land.

In the past, people relied on the clay brought by floods. The Nile flood is a famous example. Every year the Nile River flooded its lower reaches in Egypt. The floods brought water, clay, and nutrients that fertilized the land. When the flood waters went down, the farmers could then grow their crops. For thousands of years this regular flooding enabled farmers to grow enough for the Egyptians to eat. However, the flooding was stopped when the Aswan High Dam was built and so the lower Nile no longer receives its annual supply of clay.