water elm vs rock elm

water elm and rock elm both are nouns.

Word	Noun	Adjective	Verb	Adverb
water elm	Yes	No	No	No
rock elm	Yes	No	No	No

As nouns, water elm and rock elm are synonyms defined as:

water elm and rock elm: large ornamental tree with graceful gradually spreading branches common in eastern North America

Other synonyms of water elm include American elm, Ulmus americana, white elm.

water elm (noun)	rock elm (noun)
large ornamental tree with graceful gradually spreading branches common in eastern North America	tall widely distributed elm of eastern North America
Eurasian elm closely resembling the American elm; thrives in a moist environment	large ornamental tree with graceful gradually spreading branches common in eastern North America