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.
例句
By 09.30 we had cast off and were heading down-river.9点30分时我们就解缆向下游出发了。...a big tourist hotel a few hundred yards down-river...下游几百码处的一家大型旅游酒店