Two-thirds of the world's major rivers have now been disrupted with more than 50,000 dams in an attempt to stole water and provide power. In the US,there are more than 85,000 dams,disrupting large and small rives, and in most cases transforming natural flow.The most famous of these,the Hoover Dam, constructed in the l930's,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 impotent sites. Environmentally, the new reservoir can be a paradise for wildlife, especially birds. However, it can cause greenhouse gas emissions and poison the water for fish. Also, the dramatic rise and fall of water levels during dam releases is too extreme for plains 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. Dams don't just block water flow. They also prevent fish migrations, and are a barrier to sediment  flows. Instead of rushing downriver, sediments get backed up against the dam wall, which cause 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 , often host to large cities,which are sinking into the oceans. Groundwater is being extracted to feed the city, causing the urban weight to sink and sediments washed away by the ocean are no longer being replaced. The result is that sea level rise in cities from Shanghai to Alexandria. What does the text mainly talk about?
Environmental benefits of dams Cultural loss of dams Economical gain of dams Environmental loss of dams.