water elm vs American elm

water elm and American elm both are nouns.

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

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

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

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

water elm (noun)	American elm (noun)
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