River Thames vs Thames River

River Thames and Thames River both are nouns.

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

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

River Thames and Thames River: the longest river in England; flows eastward through London to the North Sea

Other synonyms of River Thames include Thames.

River Thames (noun)	Thames River (noun)
the longest river in England; flows eastward through London to the North Sea	the longest river in England; flows eastward through London to the North Sea