Sneaky !!
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The construction of Glenn Canyon Dam on the Colorado river has caused unforeseen environmental effects. By preventing annual floods, the dam prevents the river from stirring up sediments collected on the river bottom and from depositing them high on the banks of the river. These sediments are necessary for the creation and maintenance of sandbars, which form natural habitats for native fish. If water is released from the dam annually, the river would flood and thus recreate the sandbars normally found below the dam.
Understand : - Dam >> prevents flood >> prevents river from stirring up seds collected on the river bottom and from depositing seds on banks
- seds>> necessary . why? >> creation and maintenance of SBs
Conclusion : Water release from dam annually >> river would flood and recreate sandbars normally found BELOW the dam.Quote:
The prediction above relies on which of the following?
- ASSUMPTION
Think : Ron Purewal suggestion : Whenever you read the argument first think what YOU KNOW about the thing discussed in argument. First use YOUR OWN KNOWLEDGE. not too much just enough. "DAM" - upstream, downstream , reservoir, electricity generation,gates. That's all i can remember.
The author says that dams have caused some damage. they prevent formation of something which is formed naturally by floods . So basicallybhis argument is : Dams>> no floods>> no sandbars. He concludes that if we release water from dam , sandbars will again happen.
will they??
1) Upstream is the water on the left side/ water stopped by the dam . Diwnstream is water released from the dam that is the right side if the dam . When the flood occurs all the sediments ( irrespective of where they are) are stirred up and are in the flow, but when there is a dam (picturise a dam)
amount of sediments on the upstream side are not all flown to the downstream side.
See when we open the gates, the water which will flow wildly will be on the downstream side as there is som resistenceon the upstream side due to the dam. So the sand formation will happen on the downstream side and not the upstream side. But what if becasue of the dam a huge amount of sediments deposits on the upstream side?? it is possible as sediments tend to settle in calm water (which is usually the case on the upstream side), then the amount available for sand bar formation MAY not be sufficient.
And what are sand bars basically ?? sediments deposited at a place right.? so if there are no adequakte amounts of seds available then, sandbars may not happen.
2) What if the flood due to the water released from dam is not upto the par for formation of sand bars??
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A. There are no other effects of the dam that would prevent the native fish of the river from using the new sandbars below the dam.
- The formation of sandbars is our concern. Native fish is not our concern, because that is a later stage.
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B. The flood caused by releasing water from the dam would be like a natural flood in every major respect.
- The reason why i said this question is sneaky !!
The term "every" makes this ans a NOT MUST BE TRUE. See even if one of the major aspects is not comparable to aspect of natural flood , then the cnclsuoon may still sand. It is not compulsory that EVERY major aspect be met. 99 out of 100 are also okay.
Assumption MUST BE TRUE. The ans choice if negated should DESTROY the conclusion . If there is a slightest chance that the conclusion may hold then ans choice is NOT AN ASSUMPTION .
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C. The sandbars that would result from the artificial flooding of the river would be exactly the same size as the sandbars previously created by normal flooding.
- Why do we want exactly the same size?? what if they are larger?? Conclusion may still hold.
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D. There are other dams upriver from the Glenn Canyon Dam that must also release water in order for the flood from the Glenn Canyon Dam to be successful.
- Musr release - suggestion is not an assumption
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E. The dam does not prevent the flow of sediments from upriver needed to replenish the sediments that inevitably wash downstream over time.
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CORRECT Read "think" above.
close
20 Nov 2018, 04:53
54%
(02:33)
wrong 
