downriver vs downstream

downriver is not an adjective while downstream is an adjective.

Word	Noun	Adjective	Verb	Adverb
downriver	No	No	No	Yes
downstream	No	Yes	No	Yes

As adverbs, downriver and downstream are synonyms defined as:

downriver (adjective)	downstream (adjective)
	in the direction of a stream's current

downriver (adverb)	downstream (adverb)
away from the source or with the current	away from the source or with the current