tributary vs contributory

tributary is a noun but contributory is not a noun.

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

As adjectives, tributary and contributory are synonyms defined as:

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

Other synonyms of tributary include conducive, contributing, contributive.

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

tributary (adjective)	contributory (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