What a loaded question. I have never heard of anyone trying this.

First of all windmills aren't the best idea because most of the oxygen supplied to the pond is from surface contact and it is supplied naturally whenever your windmill would be running. Second when the wind is calm on a hot cloudy evening you are looking for trouble. Windmills supply aeration at the same time that mother nature would be doing it anyway.

As for running the pump off the stream it would depend on the flow rate and volume. I think it would be very hard convert it. The air produces a lot higher RPM than the water flow would. I have survived 100 mph winds and of course water moves at a much slower rate. There would have to be some sort of gear box for the reduction.

Seems like a big project.

Brian