"There is no difference, it's the relative motion between the wind and water that counts."
"It seems to me that the relative speed between wind and water surface is what matters but I'll keep thinking about it."
I tend to agree, but does'nt the river moving at a given velocity have about a thousand times more momentum then the air ,and 1/2 that factor in KE?(KE = 1/2 mv" where "=2 ??) for the same wind speed?
Think about when kiting and stopped in the river, does 15knots of wind drive you up river or does 5knots of current drive you down river( upwind)?
20knots of apparent wind but you get driven into the wind by the current?