river boulder vs bowlder

river boulder and bowlder both are nouns.

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

As nouns, bowlder is a hypernym of river boulder; that is, bowlder 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
bowlder: a large smooth mass of rock detached from its place of origin

Other hypernyms of river boulder include boulder.

river boulder (noun)	bowlder (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