What is a floodplain, and why did farmers love living on one?

A floodplain is the flat land beside a river that the river covers when it runs high, and farmers loved it because every flood leaves fresh, nutrient-rich mud behind. Floodplains stretch from the river's banks to the foot of the valley, and regular flooding gives them high soil fertility because nutrients are deposited with the floodwater. That is why some of the most important farming regions, such as the Nile and Mississippi basins, heavily use their floodplains.

A flood starts when a river carries more water than its channel can hold and spills over the banks. The extra water can come from sustained rainfall, rapid snowmelt, monsoons or tropical cyclones. As the water spills across the plain, it leaves a thin layer of sediment, coarsest and thickest close to the channel. Over the years, rapid deposition builds natural levees, ridges along the banks. In an undisturbed river, spilling over the banks is not a disaster but a regular event, happening about once every one to two years.

The benefits were real. Smaller and more frequent floods recharge groundwater and make soil more fertile, and in dry regions the floodwater supplies water that is badly needed and kills pests in the fields. Periodic flooding was essential to ancient peoples along the Tigris-Euphrates, the Nile, the Indus, the Ganges and the Yellow River. People in many countries have long lived by rivers because the land is flat and fertile.

But the same bargain has a price. Some of the worst natural disasters in history, measured by deaths, have been river floods, especially on China's Yellow River. The 1887 flood there killed around one million people. The risk is why people keep trying to control the water.