contributory vs tributary

contributory is not a noun while tributary is a noun.

Word	Noun	Adjective	Verb	Adverb
contributory	No	Yes	No	No
tributary	Yes	Yes	No	No

As adjectives, contributory and tributary are synonyms defined as:

contributory and tributary: tending to bring about; being partly responsible for

Other synonyms of contributory include conducive, contributing, contributive.

contributory (noun)	tributary (noun)
	a branch that flows into the main stream

contributory (adjective)	tributary (adjective)
tending to bring about; being partly responsible for	tending to bring about; being partly responsible for
	paying tribute
	(of a stream) flowing into a larger stream