Introduction                                                     

A dam is built to trap water, usually in a valley where there is an existing lake.

Water is allowed to flow through tunnels in the dam, to turn turbines and thus drive generators.

Notice that the dam is much thicker at the bottom than at the top, because the pressure of the water increases with depth.

Hydro-electric power stations can produce a great deal of power very cheaply.

When it was first built, the huge "Hoover Dam", on the Colorado river, supplied much of the electricity for the city of Las Vegas; however now Las Vegas has grown so much, the city gets most of its energy from other sources.

There's a good explanation of how hydro power works at www.fwee.org.

Although there are many suitable sites around the world, hydro-electric dams are very expensive to build.

However, once the station is built, the water comes free of charge, and there is no waste or pollution.

• The dams are very expensive to build.
  However, many dams are also used for flood control or irrigation, so building costs can be shared.
  
  
• Building a large dam will flood a very large area upstream, causing problems for animals that used to live there.
  
  
• Finding a suitable site can be difficult - the impact on residents and the environment may be unacceptable.
  
  
• Water quality and quantity downstream can be affected, which can have an impact on plant life.
  


• For more information about how economic factors might be pushing too hard to get dams built, see the BBC news site.