River Arno vs Arno River

River Arno and Arno River both are nouns.

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

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

River Arno and Arno River: a river in central Italy rising in the Apennines and flowing through Florence and Pisa to the Ligurian Sea

Other synonyms of River Arno include Arno.

River Arno (noun)	Arno River (noun)
a river in central Italy rising in the Apennines and flowing through Florence and Pisa to the Ligurian Sea	a river in central Italy rising in the Apennines and flowing through Florence and Pisa to the Ligurian Sea