South Platte River vs South Platte

South Platte River and South Platte both are nouns.

Word	Noun	Adjective	Verb	Adverb
South Platte River	Yes	No	No	No
South Platte	Yes	No	No	No

As nouns, South Platte River and South Platte are synonyms defined as:

South Platte River (noun)	South Platte (noun)
a tributary of the Platte River	a tributary of the Platte River