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