Floods can occurin rivers when the flow rate exceeds the capacity of the river channel,particularly at bends or meanders in the waterway. Floods often cause damage tohomes and businesses if they are in the natural flood plains of rivers. Whileriverine flood damage can be eliminated by moving away from rivers and otherbodies of water, people have traditionally lived and worked by rivers becausethe land is usually flat and fertile and because rivers provide easy travel andaccess to commerce and industry.
A
Fire and flood aretwo of humanity's worst nightmares. People have, therefore, always sought tocontrol them. Forest fires are snuffed out quickly. The flow of rivers isregulated by weirs and dams. At least, that is how it used to be. But forestershave learned that forests need fires to clear out the brash and even to getseeds to germinate. And a similar revelation is now - dawning on hydrologists.Rivers - and the ecosystems they support - need floods. That is why a man-madetorrent has been surging down the Grand Canyon. By Thursday March 6th it wasrunning at full throttle, which was expected to be sustained for 60hours.
B
Floods once ragedthrough the canyon every year. Spring Snow from as far away as Wyoming wouldmelt and swell the Colorado river to a flow that averaged around 1,500 cubicmetres (50,000 cubic feet) a second. Every eight years or so, that figurerose to almost 3,000 cubic metres. These floods infused the river withsediment, carved its beaches and built its sandbars.
C
However, in thefour decades since the building of the Glen Canyon dam, just upstream of theGrand Canyon, the only sediment that it has collected has come from tiny,undammed tributaries. Even that has not been much use as those tributaries arenot powerful enough to distribute the sediment in an ecologically valuableway.
D
This lack offlooding has harmed local wildlife. The humpback chub, for example, thrived inthe rust-red waters of the Colorado. Recently, though, its population hascrashed. At first sight, it looked as if the reason was that the chub werebeing eaten by trout introduced for sport fishing in the mid-20th century. Buttrout and chub co-existed until the Glen Canyon dam was built, so somethingelse is going on. Steve Gloss, of the United States' Geological Survey (USGS),reckons that the chub's decline is the result of their losing their mostvaluable natural defense, the Colorado's rusty sediment. The chub were welladapted to the poor visibility created by the thick, red water which gave theriver its name, and depended on it to hide from predators. Without the cloudywater the chub became vulnerable.
E
And the chub arenot alone. In the years since the Glen Canyon dam was built, several specieshave vanished altogether. These include the Colorado pike-minnow, the razorbacksucker and the round-tail chub. Meanwhile, aliens including fathead minnows,channel catfish and common carp, which would have been hard, put to survive inthe savage waters of the undammed canyon, have moved in.
F
So flooding is theobvious answer. Unfortunately, it is easier said than done. Floods were sentdown the Grand Canyon in 1996 and 2004 and the results were mixed. In 1996 theflood was allowed to go on too long. To start with, all seemed well. The floodwatersbuilt up sandbanks and infused the river with sediment. Eventually, however,the continued flow washed most of the sediment out of the canyon. This problemwas avoided in 2004, but unfortunately, on that occasion, the volume of sandavailable behind the dam was too low to rebuild the sandbanks. This time, theUSGS is convinced that things will be better. The amount of sediment availableis three times greater than it was in 2004. So if a flood is going to do somegood, this is the time to unleash one.
G
Even so, it mayturn out to be an empty gesture. At less than 1,200 cubic metres a second, thisflood is smaller than even an average spring flood, let alone one of themightier deluges of the past. Those glorious inundations moved massivequantities of sediment through the Grand Canyon, wiping the slate dirty, andmaking a muddy mess of silt and muck that would make modern river rafterscringe.
Questions1-7
Do the followingstatements agree with the information given in Reading Passage?
In boxes 1-7 onyour answer sheet, write
TURE FALSE
NOT GIVEN
if the statementagrees with the information if the statement contradicts the information
if there is noinformation on this
1 Damagecaused by fire is worse than that caused by flood.
2 The floodpeaks at almost 1500 cubic meters every eight years.
3 Contribution of sediments delivered by tributaries has littleimpact.
4 Decreasingnumber of chubs is always caused by introducing of trout since mid 20thcentury.
5 It seemedthat the artificial flood in 1996 had achieved success partly at the verybeginning.
6 In fact,the yield of artificial flood water is smaller than an average natural flood atpresent.
7 Mightyfloods drove fast moving flows with clean and high quality water.
Questions 8-13
Complete thesummary below.
Choose NO MORE THAN TWO WORDS from the passage for each answer. Writeyour answers in boxes 8-13 on your answer sheet.
The eco-impact ofthe Canyon Dam
Floods arepeople's nightmare. In the past, canyon was raged by flood every year. The snowfrom far Wyoming would melt in the season of 8........................ andcaused a flood flow peak in Colorado river. In the four decades after peoplebuilt the Glen Canyon dam, it only could gather 9........................together from tiny, undammed tributaries.
Humpback chubpopulation on reduced, why?
Then, severalspecies disappeared including Coloradopike-minnow, 10........................ and the round-tail chub.Meanwhile, some moved in such as fathead minnows, channel catfishand 11........................ . The non-stopped flow leaded to thewashing away of the sediment out of the canyon, which poses great threat to thechubs because it has poor12........................ away from predators. Inaddition, the volume of 13........................ available behind thedam was too low to rebuild the bars and flooding became more serious.
答案解析：
1 NOT GIVEN
【原文参考依据——A段首句】
Fire and flood are two of humanity's worstnightmares. 文中只提到了火灾和洪水是人类最糟糕的两类噩梦，并没有说哪一个导致的破坏更大的问题。
2 FALSE
【原文参考依据——B段第二至三句】SpringSnow from as far away as Wyoming would melt and swell the Colorado river to aflow that an averaged around 1,500 cubic metres (50,000 cubic feet) a second.Every eight years or so, that figure rose to almost 3,000 cubic metres.
3 NOT GIVEN
【原文参考依据——C段】
C段中并没有提及沉积物的贡献问题
4 FALSE
【原文参考依据——D段第三行至第六行】Atfirst sight, it looked as if the reason was that the chub were being eaten bytrout introduced for sport fishing in the mid-20th century. But trout and chubco-existed until the Glen Canyon dam was built, so something else is going on. 初看之下，以为是trout的原因，但是trout和chub自大坝建成以来就一直共存，可见是有其他原因造成chub的数量锐减。
5 TRUE
【原文参考依据——F段第四行】
To start with, all seemed well.
6 TRUE
【原文参考依据——G段第二句】Atless than 1,200 cubic metres a second, this flood is smaller than even anaverage spring flood, let alone one of the mightier deluges of the past.
7 NOT GIVEN
【原文参考依据——无】
文章并没有提到相关内容
8 spring
【原文参考依据——B段第二句】SpringSnow from as far away as Wyoming would melt and swell the Colorado river to aflow that averaged around 1,500 cubic metres (50,000 cubic feet) a second.
9 sediment
【原文参考依据——C段首句】However,in the four decades since the building of the Glen Canyon dam, just upstream ofthe Grand Canyon, the only sediment that it has collected has come from tiny,undammed tributaries.
10 razorback sucker
【原文参考依据——E段第三句】
These include the Colorado pike-minnow, therazorback sucker and the roundtail chub.
11 common carp
【原文参考依据——E段末句】Meanwhile,aliens including fathead minnows, channel catfish and common carp, ……
12 visibility
【原文参考依据——D段倒数第二句】Thechub were well adapted to the poor visibility created by the thick, red waterwhich gave the river its name, and depended on it to hide from predators.
13 sand
【原文参考依据——F段第十行】……the volume of sand available behind the dam was toolow to rebuild the sandbanks.
