river vs Big Sioux River

river and Big Sioux River both are nouns.

Word	Noun	Adjective	Verb	Adverb
river	Yes	No	No	No
Big Sioux River	Yes	No	No	No

river (noun)	Big Sioux River (noun)
a large natural stream of water (larger than a creek)	a river that rises in northeastern South Dakota and flows southward to join the Missouri River at Sioux City, Iowa