It is still an amazing thing, no wind at all, just the current flowing lets say South 10 knots.
Then you as a kitesurfer in the water, will feel 10 knots of wind from South going North.
Meaning, when you are in the water you drift with 10 knots going South, and when launching (with the kite North of yourself) and riding you can ride "downwind" as fast as you can, and as we know, you can have a higher downwind (and some also upwind) VMG than the 10 knots of "wind", so you can ride against the current going "downwind" and end up going more and more to North relative to land, when you are really fast
It would be pretty special though, just to ride when zero wind and fast current only, as the wind gradient in this scenario, would be much much lower than usual over land, and the "wind" would also be incredible smooth and close to "perfect", no turbulence at all - WOW
It would not matter if high trees or canyons or buildings around the river, as it would not disturb this perfect "wind" at all, really a paradoxial scenario
Also, if you did not ride a fast hydrofoil, but launched your kite, and looped it bodydragging at full pace "downwind" (towards North), you would still end up further south from your starting point no matter how good you are at looping and getting dragged really fast "dead downwind"