alluvial plain vs alluvial flat

alluvial plain and alluvial flat both are nouns.

Word	Noun	Adjective	Verb	Adverb
alluvial plain	Yes	No	No	No
alluvial flat	Yes	No	No	No

As nouns, alluvial plain and alluvial flat are synonyms defined as:

alluvial plain and alluvial flat: a flat resulting from repeated deposits of alluvial material by running water

alluvial plain (noun)	alluvial flat (noun)
a flat resulting from repeated deposits of alluvial material by running water	a flat resulting from repeated deposits of alluvial material by running water