A dangling construction are misplaced or wrongly organized words that the writer intends to be a modifier , but instead of doing that, the modifier erroneously describes a noun or phrase that is in the sentence -- sometimes with humorous results. In this case, the problem is that in this sentence the Pouring millions of gallons over the waterfall seems to be done by Bill.
The correct structure would be:
Bill marveled at Niagara Falls pouring millions of gallons over the waterfall.
A fable is a short piece of fiction that features animals in the role of the protagonist and usually includes or illustrates a moral. The distinguishing feature of a fable is the anthropomorphism or personification involved that leads to a moral lesson being taught.