flood plain

Area of periodic flooding along the course of a river valley. When river discharge exceeds the capacity of the channel, water rises over the channel banks and floods the surrounding low-lying lands. As water spills out of the channel some alluvium (silty material) will be deposited on the banks to form levees (raised river banks). This water will slowly seep onto the flood plain, depositing a new layer of rich fertile alluvium as it does so. Many important flood plains, such as the inner Nile delta in Egypt, are in arid areas where their exceptional productivity is very important to the local economy.

A flood plain is a natural feature, flooded at regular intervals. By plotting floods that have occurred we can speak of the size of flood we would expect once every 10 years, 100 years, 500 years, and so on.

Even the most energetic flood-control plans (such as dams, dredging, and channel modification) sometimes fail, and if towns and villages are built on a flood plain there is always some risk. It is wiser to use flood plains in ways that are compatible with flooding, such as for agriculture or parks.

Flood-plain features include meanders and oxbow lakes.