Styx vs River Styx

Styx and River Styx both are nouns.

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

As nouns, Styx and River Styx are synonyms defined as:

Styx and River Styx: (Greek mythology) a river in Hades across which Charon carried dead souls

Styx (noun)	River Styx (noun)
(Greek mythology) a river in Hades across which Charon carried dead souls	(Greek mythology) a river in Hades across which Charon carried dead souls