river vs Mekong River

river and Mekong River both are nouns.

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

river (noun)	Mekong River (noun)
a large natural stream of water (larger than a creek)	an Asian river; flows through a large delta in southern Vietnam into the South China Sea