River Acheron vs river

River Acheron and river both are nouns.

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

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