river vs River Acheron

river and River Acheron both are nouns.

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

river (noun)	River Acheron (noun)
a large natural stream of water (larger than a creek)	(Greek mythology) a river in Hades across which the souls of the dead were carried by Charon