A typical Dam

Dam = any obstruction placed in a river or stream to block the flow of water so that water can be stored in a reservoir
To prevent floods, provide drinking water, allow irrigation, and generate electricity
45,000 large dams have been erected in more than 140 nations
Only a few major rivers remain undammed
In remote regions of Canada, Alaska, and Russia

Major Benefits and drawbacks of dams

Benefits:
Power generation
Emission reduction
Crop irrigation
Drinking water
Flood control
Shipping
New recreational opportunities

Drawbacks:
Habitat alteration
Fisheries declines
Population displacement
Sediment capture
Disruption of flooding
Risk of failure
Lost recreational opportunities