15．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 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．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．
51．What effects does creating a reservoir have on people's life？D
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．a，c B．c，e C．b，e D．b，c
52．With many dams built across rivers，fish willB
A．find more places to lay their eggs B．face a more dangerous living habitat
C．feed on more sediment rich in nutrients D．find it easier to deal with the rise of water
53．How can dams influence the cities like Shanghai？C
A．Its entire system will be influenced． B．The rainy season will last longer．
C．It will lie below sea level in the future D．Rich soils will be washed away
54．What does the text mainly talk about？A
A．Environmental loss of dams．B．Cultural loss of dams
C．Economical gain of dams D．Environmental benefits of dams
55．Where can we most probably read this passage？C
A．In a travel magazine B．In a personal diary
C．In a science report D．In a biology textbook．
分析 本文属于说明文阅读，作者通过这篇文章主要向我们描述的是人类在世界各地修建的大坝对环境造成的影响．它不仅淹没了一些文化遗迹，还导致下游的农民无法灌溉农作物，更使得三角洲的一些大城市面临被海水淹没的危险，因此对于大坝的建造一定要谨慎，不要因一时的利益造成更大的损失．
解答 51．D．细节理解题．根据文章第二段"Communities may lose their land，houses and culturally impotent sites"和第四段"The fertility of the entire system can be influenced，with soils lost during seasonal rains not being replaced"可知，随着大坝的修建，蓄水淹没了一些文化遗迹，还导致下游的农民无法灌溉农作物，故选D．
52．B．推理判断题．根据文章第三段中的"it can cause greenhouse gas emissions and poison the water"可知，随着大坝的修建，蓄水会释放出温室气体，使水有毒，所以鱼的生活环境会变得危险，故选B．
53．C．细节理解题．根据文章最后一段中的"Perhaps the biggest problem can be seen in deltas（三角洲），often host to large cities，which are sinking into the oceans"可知，三角洲中的大型城市可能会被海水淹没，上海正属于位于三角洲的大型城市，因此很可能被海水淹没，故选C．
54．A．主旨大意题．通读全文可知，本文主要讲的是修建大坝对环境造成的影响，它淹没了一些文化遗迹，还导致下游的农民无法灌溉农作物，更使得三角洲的一些大城市面临淹没的危险，可知主旨为大坝对环境的危害；故选A．
55．C．推理判断题．通读全文可知本文主要讲的是修建大坝对环境造成的影响，给出了实例充分说明了这个观点，语言十分严谨，可推出这是一份科学报告；故选C．
点评 考察学生的细节理解和推理判断能力，做细节理解题时一定要找到文章中的原句，和题干进行比较，再做出正确的选择．在做推理判断题不要以个人的主观想象代替文章的事实，要根据文章事实进行合乎逻辑的推理判断．
