A dam blocks the flow of a stream or river. This blockage forms a reservoir, or artificial lake. The first known dam was built in Egypt around 3000 BCE.
In the last hundred years, many more dams have been built. More than 45,000 major dams have been built in 150 countries. Dams have changed 60 percent of the world’s rivers. This redistribution of water has actually changed the way Earth wobbles as it spins.
Dams do many great things: they reduce flooding, make electricity, get water to farms, and supply drinking water for millions of people.
In recent years, it has become clear that dams also do a lot of harm. Creating artificial lakes floods homes, forcing many people to move. This also destroys forests and farmland, which kills many native species. Building dams also creates pollution.
