rock elm vs water elm

rock elm and water elm both are nouns.

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

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

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

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

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