Two-thirdsoftheworld’smajorrivershavenowbeendisrupted(中...

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Two-thirds of the world’s major rivers have now been disrupted(中断)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(水库) 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(排放)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. The flow may be reduced so that farmers will have great trouble getting water for their fields. They also prevent fish migrations, and are a barrier to sediment (沉淀物)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(三角洲), 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.

28. What effects does creating a reservoir have on people’s life?

a. People will see fewer birds near the rivers．

b. Farmers below the dam may have less water to irrigate their fields

c. Visitors can’t see some cultural relics anymore．

d. More conflicts and wars over water will break out

e. More kinds of fish will become extinct in the future

A. b, c B. c, e C. b, e D. a, c

29. With many dams built across rivers, fish will_______

A. find more places to lay their eggs B. feed on more sediment rich in nutrients

C. face a more dangerous living habitat D. find it easier to deal with the rise of water

30. How can dams influence the cities like Shanghai?

A. Its entire system will be influenced. B. It will lie below sea level in the future