The construction of Glenn Canyon Dam on the Colorado river : GMAT Critical Reasoning (CR)
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# The construction of Glenn Canyon Dam on the Colorado river

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14 Jul 2011, 20:12
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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.

The prediction above relies on which of the following?

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 flood caused by releasing water from the dam would be like a natural flood in every major respect.
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.
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.
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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14 Jul 2011, 20:38
bschool83 wrote:
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.

The prediction above relies on which of the following?

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. - Negate this, and the argument is not affected.

The flood caused by releasing water from the dam would be like a natural flood in every major respect. - Negate this, and the argument falls apart. If water relase is not like a natural flood, sandbars will not be formed

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. Negate this, and the argument is not affected. How about other attributes of sandbars as durability, strength etc. ?

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. Negate this, and the argument is not affected.

The dam does not prevent the flow of sediments from upriver needed to replenish the sediments that inevitably wash downstream over time. Negate this, and the argument is not affected.

What is the OA ?

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14 Jul 2011, 20:44
B.
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14 Jul 2011, 20:59
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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. If you look carefully at the conclusion, it just says that release of water --> Flood --> Recreation of sand bars. It does NOT say that the fish would come and stay in THESE sandbars. Thus the prediction, as a standalone statement, doesnt per se depend on whether the fish will use these sandbars or not
The flood caused by releasing water from the dam would be like a natural flood in every major respect. The predictios is saying release of water will cause a flood, which it assumes will create sandbars like a normal natural flood does. this is important for the prediction to stand - if the dam flood doesnt move sediments it wont create sandbars and the prediction would fail!
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. Similar to A, creation of sandbar is important, whether they are usable/ same or different doesnt really matter at least for this prediction
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.The prediction doesnt rely on that - it doesnt depend on any other dams
The dam does not prevent the flow of sediments from upriver needed to replenish the sediments that inevitably wash downstream over time.If this were true, the prediction would fail..if the dam is not preventing the sediments from coming down and creating sandbars anyway, what is the need of the flood?
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15 Jul 2011, 03:15
Nicely explained. Thanks.
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15 Jul 2011, 05:51
B is incorrect!

I'm happy that the majority of the people got this one wrong. I feel that my learning is maximized when I have made a mistake. Sir Winston Churchill once said... "All men make mistakes, but only wise men learn from their mistakes".

I like this question because it has a lot of trap answers. So let's systematically focus on the stimulus. In an assumption question, look for an unstated information that is required for the argument to be true.

PARAPHRASING is key here.

Evidence: Natural floods stir up sediments creating natural sandbars providing a habitat for fish. Dams prevent stirring up of sedimentation.
Conclusion: Artificial flooding would recreate sandbars.

Now start to think of some unstated assumptions without looking at the answers:
1. artificial floods can stir up the sediments the same way natural floods do
2. sediments still exist in the bottom of the dam

Clearly E matches our assumption 2. Always important to think through some thoughtful assumptions first before looking at the answers.

OA is E.

Let me know if you need any further explanation.
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21 Jul 2011, 12:46
i chose C...curious to know why C is wrong?
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21 Jul 2011, 12:53
LifeChanger wrote:
i chose C...curious to know why C is wrong?

Why do you think size of sandbars matter?
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22 Jul 2011, 06:45
bschool83 wrote:
LifeChanger wrote:
i chose C...curious to know why C is wrong?

Why do you think size of sandbars matter?

Argument says: sandbars, which form natural habitats for native fish

so,if the native fish were to have a natural habitat during artificial flooding, the situation would be better when sandbars formed are of the same size
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22 Jul 2011, 08:23
LifeChanger wrote:
bschool83 wrote:
LifeChanger wrote:
i chose C...curious to know why C is wrong?

Why do you think size of sandbars matter?

Argument says: sandbars, which form natural habitats for native fish

so,if the native fish were to have a natural habitat during artificial flooding, the situation would be better when sandbars formed are of the same size

This is a classic example of bringing external or extra information into the argument. Please refrain from doing so. If the argument does not mention the size of sandbars, then an assumption about the size of sandbars is out of scope. Work with only the given information.

Also if you think critically does same size mean identical? If two buildings are 100 ft tall, are they identical?

Hope this helps.
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23 Jul 2011, 01:03
This is a classic example of bringing external or extra information into the argument. Please refrain from doing so. If the argument does not mention the size of sandbars, then an assumption about the size of sandbars is out of scope. Work with only the given information.

Also if you think critically does same size mean identical? If two buildings are 100 ft tall, are they identical?

Hope this helps.[/quote]
Valid point..Thanks for reminding it
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Re: The construction of Glenn Canyon Dam on the Colorado river [#permalink]

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05 Mar 2014, 05:26
Quote:
Now start to think of some unstated assumptions without looking at the answers:
1. artificial floods can stir up the sediments the same way natural floods do
2. sediments still exist in the bottom of the dam

Quote:
The dam does not prevent the flow of sediments from upriver needed to replenish the sediments that inevitably wash downstream over time.

Sorry but I am unable to follow your second assumption.This means that the effect of dam that the author is stating is not actually there, which gives a fairly possible explaination of why the flooding would not help replenishing the sediments. But if you look at B
Quote:

If you negate this then again the conclusion that the release of water will lead to formation of sand bars, will fall. Also it follows the first assumption that you (and I as well ) made

So do we have more than one answer choice here which can break the conclusion and hence is the right answer? Can this happen in real GMAT as well?
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Re: The construction of Glenn Canyon Dam on the Colorado river [#permalink]

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05 Mar 2014, 07:22
Option B.
If the flood is not natural,it wouldn't lead to proper sedimentation.

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Re: The construction of Glenn Canyon Dam on the Colorado river [#permalink]

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07 Mar 2014, 01:05
Hi,

I would like to add my 2 cents here.
The main point in this CR argument is reading the question wisely.

What is the question actually asking.

The prediction above relies on which of the following?

Most of us would read ...OK prediction...find an ASSUMPTION question.But the question is really a tricky one. It consider the whole argument as a prediction and asks that IT RELIES ON WHAT???

Clearly if u guys understand the question the correct answer would come out as E. i.e. the dam doesnt PREVENT the flow of sediments from upstream to downstream...which is necessary for the habitat to flourish.

Just the water flowing down wouldnt help.

Hope it clarifies.

-h.
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Re: The construction of Glenn Canyon Dam on the Colorado river [#permalink]

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16 Mar 2014, 08:19
I think the answer is 'E'. May be I am wrong, but 'B' seems to be a bit out of scope.
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Re: The construction of Glenn Canyon Dam on the Colorado river [#permalink]

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17 Mar 2014, 01:50
I believe that it is a close finish between B & E. I would go with E.

B. The flood caused by releasing water from the dam would be like a natural flood in every major respect.
B could be correct but there is not definition of the term major aspect. One would have to assume that carrying the sediment would come under the 'every major respect' mentioned in option B.

E. The dam does not prevent the flow of sediments from upriver needed to replenish the sediments that inevitably wash downstream over time.
E, however, stays within the scope of the argument taking into account the premise that sediments are needed for these sand beds and that once water is released the flow of sediments will not be restricted by the dam.

An expert's opinion on this topic would be most helpful. Any experts feeling generous today?
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Re: The construction of Glenn Canyon Dam on the Colorado river [#permalink]

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17 Mar 2014, 10:44
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Hi - Not sure if I count as an expert, but this is my view:

The answer to this one is E.

Here is how I thought about it:

Pre dam the situation was: Flood occured & sediments built the sand banks

Post dam: Not happening.

By opening the dam: Floods occur.

So what are you missing? The sediments.

E gives you the sediments so solves the issue.

The issue with B is that it is too strong. It doesn't need to be like a natural flood in EVERY way - just that it gives the sediments. It could easily differ in a different manner (e.g. last a different length of time, happen at a different time of year) and the sand banks would still be OK.

Hope that helps

James
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Re: The construction of Glenn Canyon Dam on the Colorado river   [#permalink] 17 Mar 2014, 10:44
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