river boulder vs boulder

river boulder and boulder both are nouns.

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

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

river boulder: a boulder that has been carried by a river to a place remote from its place of origin
boulder: a large smooth mass of rock detached from its place of origin

Other hypernyms of river boulder include bowlder.

river boulder (noun)	boulder (noun)
a boulder that has been carried by a river to a place remote from its place of origin	a large smooth mass of rock detached from its place of origin