river vs River Lethe

river and River Lethe both are nouns.

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

river (noun)	River Lethe (noun)
a large natural stream of water (larger than a creek)	(Greek mythology) a river in Hades; the souls of the dead had to drink from it, which made them forget all they had done and suffered when they were alive