Something that is moving down-river is moving towards the mouth of a river, from a point further up the river. Something that is down-river is towards the mouth of a river.

Which way is up river and down river?

As adverbs the difference between downriver and upriver

is that downriver is travelling in the direction of the river current while upriver is towards the source of a river.

What is upriver or downriver?

upriver - toward the source or against the current. upstream. downriver, downstream - away from the source or with the current.