river vs Milk River

river and Milk River both are nouns.

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

river (noun)	Milk River (noun)
a large natural stream of water (larger than a creek)	a river that rises in the Rockies in northwestern Montana and flows eastward to become a tributary of the Missouri River