river vs Danube River

river and Danube River both are nouns.

Word	Noun	Adjective	Verb	Adverb
river	Yes	No	No	No
Danube River	Yes	No	No	No

river (noun)	Danube River (noun)
a large natural stream of water (larger than a creek)	the 2nd longest European river (after the Volga); flows from southwestern Germany to the Black Sea