river vs River Styx

river and River Styx both are nouns.

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

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