The lake behind a dam is called a reservoir. Some of the water from the river must be allowed to pass through the dam. If this didn't happen, the river would dry up. Also, the reservoir would grow until it flowed over the top of the dam.

The purposes of a dam is to provide water for crops, water for a town or city, improving boat navigation, making electricity, flood control, making a habitat for fish and wildlife and creating recreational boating areas.

This is a satellite view of the famous Aswan dam in Egypt. It was built across the Nile River. You can see the large reservoir that it created. Professor Nibble walked across this dam. It was very exciting. Dams do many good things. However, there are some bad things that happen when dams are built too. The land behind the dam is flooded. Many historical Egyptian ruins were lost under the reservoir the Aswan Dam created.

Related Terms
reservoir - created by a dam
river - the landform that a dam controls
lake - created by a dam