From your example, one could envision an extended edition where, shortly after the shoe goes out of view[1] floating past him, someone shouts "anyone missing a shoe?"
That response would then come from downstream from Darkly.
My point was that it's a matter of perspective. And it requires a certain well-defined gradient (the river), I don't think "upstream" and "downstream" make much sense when all you have is a lake.
I just envision being Darkly Noon, sitting at the river bed, watching the giant shoe floating into view from upstream, going downstream.
Anything else don't make sense.