Any thesis or argument within a paper must be backed up with appropriate evidence. In this case, your thesis is that dammed rivers are beneficial to people. As the focus of the original essay is on Lake Powell and the Glen Canyon Dam, it would be a good idea to look at the benefits the original project and the resulting lake had on the populace.
One clear benefit that you could argue for in your essay is the economic success of rivers and dams. While Lake Powell provides a small amount of water to urbanized areas, the greater amount of water from the lake—about 85%—is used in irrigation. Because of this irrigation, states surrounding the lake are more agriculturally viable, something that they would not be without this water because of their locations in the Southwest.
Another argument could be made for the social benefits of water from the Glen Canyon Dam project. People of all kinds benefit from these lake waters, among them the Native American people who are now able to receive...
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