Gorge groundspeeds?

Just curious here about how downwinding against the current works? I've been to the Columbia river many times, and while its not especially swift, there is definitely a good current.

When people are surfskiing there is it like half the groundspeed as normal or what?

It depends on the time of year. In the spring, the river level is high, sometimes in the 4 mph range, but by now it's probably only a bit over 1 mph. Last year Kenny Rice won it in 1:32 and I think those who used a GPS showed right around 14 miles.