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.
By 09.30 we had cast off and were heading down-river.
...a big tourist hotel a few hundred yards down-river.
Cologne is not so very far down-river from Mainz.
...downriver factories dispensing billows of smoke.
COBUILD Advanced English Dictionary. Copyright © HarperCollins Publishers