River Severn vs river

River Severn and river both are nouns.

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

River Severn (noun)	river (noun)
a river in England and Wales flowing into the Bristol Channel; the longest river in Great Britain	a large natural stream of water (larger than a creek)