river vs River Arno

river and River Arno both are nouns.

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

river (noun)	River Arno (noun)
a large natural stream of water (larger than a creek)	a river in central Italy rising in the Apennines and flowing through Florence and Pisa to the Ligurian Sea