A dam is a barrier placed around water to keep it ponded up inside.

Many dams are placed across valleys to make reservoirs, or sometimes to control the flow of water so that areas further down the river will not flood.

Dams can be made from concrete, but many of the biggest dams are made of soil banks with watertight clay in the middle.

Most concrete dams are thin and tall, whereas most soil dams are broad and low. The picture shows a concrete dam.

Dams often let water out in a controlled way to keep the flow of a river steady throughout the year. They also release water for irrigating farmland, and they sometimes send water through turbines to make electricity (hydro-electric power).