Something that is moving downstream is moving towards the mouth of a river, from a point further up the river. Something that is downstream is further towards the mouth of a river than where you are.
例句
We had drifted downstream.我們漂向了下游。...a district called Lupitu, which in fact was downstream of the dam...叫勒普頓的地區，實際上在大壩的下游。
英漢詞典釋義
adv.
在下游;順流地
Downstream the river twists and turns a lot.這條河的下游彎彎曲曲。We were rowing downstream towards the sea.我們劃船順流而下, 駛向大海。