Answer:
Because the author saw and studied the area both before and after the dam was built.
Explanation:
This is the reason why the author Edward Abbey feels that he is qualified to evaluate how the land has changed after the building of the Glen canyon dam. The author tells us that he had known the area before the dam was built, and had spent considerable time there. We also learn that he studied the area after the dam was built. This knowledge contributes to the credibility of the author when talking about how the land in the area has changed.
