Complex sentence: Since its first proposal in the 19th century, the dam was highly controversial because it would be located inside the recently established Yosemite National Park, with its reservoir flooding a scenic canyon then compared to Yosemite Valley for its natural beauty.
Split into simpler sentences: Since its first proposal in the 19th century, the dam was highly controversial because it would be located inside the recently established Yosemite National Park. Its reservoir would flood a scenic canyon then compared to Yosemite Valley for its natural beauty.