Niagara River vs Niagara

Niagara River and Niagara both are nouns.

Word	Noun	Adjective	Verb	Adverb
Niagara River	Yes	No	No	No
Niagara	Yes	No	No	No

As nouns, Niagara River and Niagara are synonyms defined as:

Niagara River and Niagara: a river flowing from Lake Erie into Lake Ontario; forms boundary between Ontario and New York

Niagara River (noun)	Niagara (noun)
a river flowing from Lake Erie into Lake Ontario; forms boundary between Ontario and New York	a river flowing from Lake Erie into Lake Ontario; forms boundary between Ontario and New York
	waterfall in Canada is the Horseshoe Falls; in the United States it is the American Falls