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,...