river dolphin vs dolphin

river dolphin and dolphin both are nouns.

Word	Noun	Adjective	Verb	Adverb
river dolphin	Yes	No	No	No
dolphin	Yes	No	No	No

As nouns, dolphin is a hypernym of river dolphin; that is, dolphin is a word with a broader meaning than river dolphin:

river dolphin: any of several long-snouted usually freshwater dolphins of South America and southern Asia
dolphin: any of various small toothed whales with a beaklike snout; larger than porpoises

river dolphin (noun)	dolphin (noun)
any of several long-snouted usually freshwater dolphins of South America and southern Asia	any of various small toothed whales with a beaklike snout; larger than porpoises
	large slender food and game fish widely distributed in warm seas (especially around Hawaii)