The Collaborative International Dictionary of English v.0.48:

Niagara \Niagara\, Niagara River \Niagara River\prop. n.
   A river flowing from Lake Erie into Lake Ontario, which forms
   the boundary between Ontario and New York.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

Niagara River
    n 1: a river flowing from Lake Erie into Lake Ontario; forms
         boundary between Ontario and New York [syn: Niagara,
         Niagara River]