river vs Mississippi River

river and Mississippi River both are nouns.

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

river (noun)	Mississippi River (noun)
a large natural stream of water (larger than a creek)	a major North American river and the chief river of the United States; rises in northern Minnesota and flows southward into the Gulf of Mexico