Rivers spend most of their time in their banks. often well below the top. But, on average about once every 3 years, so much water enters the river from storms or a long, wet winter, that the rivers fill up and overtop their banks.

When this happens they flood. Flooding can also happen because the sea level rises, and water is driven into estuaries by very high tides and gales.

When flooding occurs, very large areas of land can be covered in water. That is because the river has formed a wide, flat strip of land to either side. It is called a floodplain, and with good reason.

In the past, people avoided building on floodplains because they stood no chance of preventing flooding. But in recent times people have built more and more on the floodplains and so they have had to build walls, either in concrete or as earthen banks, along the rivers to raise the height of the banks.

It is likely that more and more such walls - called dykes, levees or embankments - will be needed in the future. It's not just because global warming might cause sea levels to rise. It's more that people now build in unsuitable places, so they bring flooding on themselves.