Severn vs River Severn

Severn and River Severn both are nouns.

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

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

Severn and River Severn: a river in England and Wales flowing into the Bristol Channel; the longest river in Great Britain

Other synonyms of Severn include Severn River.

Severn (noun)	River Severn (noun)
a river in England and Wales flowing into the Bristol Channel; the longest river in Great Britain	a river in England and Wales flowing into the Bristol Channel; the longest river in Great Britain
a river in Ontario that flows northeast into Hudson Bay