Critical Reasoning

The contruction of Glenn river 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 depositing them high on the banks of the river. These sediments are neccessary for the creation and maintanance of sandbars which form natural habitats for native fish. If the 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?

A. There are no other effects of the dam that would prevent the native fish of the river from using the sandbars below the dam

B.The flood caused by releasing water from the dam would be like a natural flood in every aspect.

C.The sandbars that would result from the artifical flooding of the river would be exactly the same size as the sandbar previously created by normal flooding

D.There are other dams upriver from the Glenn Canyon Dam that must also release water in order for the flood from the Glen Canyon dam to be successful.

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

Can somebody explain why E is the answer I don't understanding the assumption here. Thanks

imo:E

explanation if its OA

OA is E its hidden behind the spoiler

...the dam prevents the river from stirring up sediments and depositing them high on the banks of the river. These sediments are neccessary for the creation and maintanance of sandbars.

The main point here is sediments.

Conclusion is
If the water is released from the dam annually the river would flood and thus recreate the sandbars normally found below the dam.

So we need to find assumption.
To recreate the sandbars normally found below the dam the flood must not prevent the flow of sediments.

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

Oh I see. When I read E I was confused. I thought the answer said the dam had another river full of sediments that does not affect the river below the dam. Thanks for the clarification.

I know the answer is E, but E is assuming that sediment is washed away over time, which is not provided in the passage. The passage does say that by preventing annual floods, the dam prevents the river from stirring up sediments collected on the river bottom, which to me leads to the conclusion that there are sediments to stir up, but the water and current is missing to stir up the sediments. The existence of the dam prevents water to be overflowing at the section of river past the dam. The section before the dam will have more water than usual, and thus is more flooded than it would have been had there not been a dam. As such, the flooding part of the river is necessarily after the dam and not before. By releasing the dam, the portion of the river past the dam would flood and stir up sediment, while the section before the dam would not necessary stir up the river bottom as water is leaving the body of water and not entering. Sediments are stirred up when a section of the river is in the process of being flooded.

So all that is needed is for the water to be released so that it floods the river. I don't quite understand why the passage needs to assume the dam does not prevent sediments from flowing through.

I think E is the right answer here.