| Noun | 1. | Niagara River - a river flowing from Lake Erie into Lake Ontario; forms boundary between Ontario and New YorkCanada - a nation in northern North America; the French were the first Europeans to settle in mainland Canada; "the border between the United States and Canada is the longest unguarded border in the world" Niagara, Niagara Falls - waterfall in Canada is the Horseshoe Falls; in the United States it is the American Falls river - a large natural stream of water (larger than a creek); "the river was navigable for 50 miles" | |