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.我们漂向了下游。...a district called Lupitu, which in fact was downstream of the dam...叫勒普顿的地区，实际上在大坝的下游。
英汉词典释义
adv.
在下游;顺流地
Downstream the river twists and turns a lot.这条河的下游弯弯曲曲。We were rowing downstream towards the sea.我们划船顺流而下, 驶向大海。