River Severn vs Severn

River Severn and Severn both are nouns.

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

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

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

Other synonyms of River Severn include Severn River.

River Severn (noun)	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