Reservoirs are most usually made for one of the following reasons:

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  To hold water for household, farming, and industrial use, especially as drinking water
• 
  To hold water to power machines, such as a water mill, or to make electricity or to store electricity
• 
  To supply water to a canal or irrigation system.
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  To hold water that would flood a community, where it can slowly and safely be released.

Reservoirs may also be used for:

• Holding fish (for food)
• Recreation - fishing and boatin

• People have to move to a new home and their property is destroyed
• Habitats are destroyed
• Large bodies of water may provide places for mosquitoes to breed
• More erosion downstream
• Fertile silts are not deposited downstream for farming
• Less water for those downstream