Erosion is the term used to describe the breakdown and removal of rock or soil from a place.

River erosion on the outside of this channel has undercut this hillside and caused it to collapse.

Erosion may happen almost instantly, as in the case of a river scraping rock from its banks and carrying it away, or it may be a very slow process, such as when rock rots, or weathers, first to form soil, and only later is carried away, or transported, perhaps by a river during a flood.

During a flood, erosion can be severe and lead to problems for people living close to a river.

River erosion has brought down huge amounts of pebbles to this mountain river. The extent of the problem was only seen when the flood waters subsided.

To help to prevent erosion steep rivers are sometimes controlled by using steps of concrete. In this example below even the boulders are cemented to the river bed to try to slow down the water that floods down the channel in spring. This picture was taken during times of low flow in summer.

Channel control to prevent erosion.