river vs Missouri River

river and Missouri River both are nouns.

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

river (noun)	Missouri River (noun)
a large natural stream of water (larger than a creek)	the longest river in the United States; arises in Montana and flows southeastward to become a tributary of the Mississippi at Saint Louis