Twothirds of the world's major rivers have now been disrupted with more than 50,000 dams in an attempt to  store water and provide power.In the US,there are more than 85,000 dams,disrupting large and small rivers,and in most cases transforming natural flow.The most famous of these,the Hoover Dam,constructed in the 1930s,is mainly responsible for the fact that the Colorado River no longer reaches the ocean. Dams,besides all their attractive benefits,also have negative impacts.Creating a reservoir means a large area must be flooded.Communities may lose their land,houses and culturally important sites. Environmentally,the new reservoir can be a paradise for wildlife,especially birds;however,it can cause greenhouse gas emissions and poison the water.Also,the dramatic rise and fall of water levels during dam releases is too extreme for plants and animals to cope with,resulting in dead zones around the shores of reservoirs.Fish that lay their eggs in the shallows,for example,may find a few hours later that those sites are high and dry. Downstream of a dam,the flow may be reduced so that farmers cannot irrigate their fields.Many rivers run through national borders,which may cause conflict over precious water.Disputes have been continuing between India and Pakistan,and Turkey,Syria and Iraq,for example. Dams don't just block the flow of water.They also prevent fish migrations,and dams are a barrier to sediment flows.Instead of rushing downriver,sediments get backed up against the dam wall,which causes the reservoir level to increase over time.However,sediments which are rich in nutrients have become a problem.The fertility of the entire system can be influenced,with soils lost during seasonal rains not being replaced. Perhaps the biggest problem can be seen in deltas,which are sinking into the oceans.Groundwater is being extracted to feed the city,causing the city to sink,and sediments washed away by the ocean are no longer being replaced.The result is that sea level rises in cities from Shanghai to Alexandria. How can dams influence large cities like Shanghai?
Their entire system will be influenced. The rainy season will last longer. They will lie below sea level in future. Rich soils will be washed away.