Americans call Niagara Falls a natural wonder, but the falls aren't very natural anymore. In fact, they are a study in artifice. Water diverted, riverbed reshaped, brink stabilized, and landscape redesigned, the falls are more a monument to man's meddling than to nature's strength. Held up as an example of something real, they are hemmed in with fakery—waxworks, haunted houses, IMAX films, and ersatz Indian tales. A symbol of American manifest destiny, they are shared politely with Canada....