Missouri River vs river

Missouri River and river both are nouns.

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

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