Big Sioux River vs river

Big Sioux River and river both are nouns.

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

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