black birch vs river birch

black birch and river birch both are nouns.

Word	Noun	Adjective	Verb	Adverb
black birch	Yes	No	No	No
river birch	Yes	No	No	No

As nouns, black birch and river birch are synonyms defined as:

black birch and river birch: birch of swamps and river bottoms throughout the eastern United States having reddish-brown bark

Other synonyms of black birch include Betula nigra, red birch.

black birch (noun)	river birch (noun)
birch of swamps and river bottoms throughout the eastern United States having reddish-brown bark	birch of swamps and river bottoms throughout the eastern United States having reddish-brown bark
common birch of the eastern United States having spicy brown bark yielding a volatile oil and hard dark wood used for furniture