Water wheels are black magic and even experts in DF physics/fluids don't always understand when they will or won't work. The "you have to avoid 7/7 water" thing is a myth. There's some poorly-understood process for deciding whether water is "flowing" for waterwheel purposes which has little or nothing to do with the processes that actually move water.
I recommend building your waterwheels directly in the river and running axles from there. Wall/roof them if you like.
You can also reliably make them work by digging a channel in an aquifer, then ponding one bucket of water into the channel from a level above to cause all the water in the channel to become "flowing". Don't ask me why that works but it always does.
In 40d I had a very reliably process of diverting underground rivers into snaking channels with waterwheels over them, and once I got the waterwheels to start turning, I could seal the input and output doors and they would keep running forever anyway.
I can also, for some reason, make a looping aqueduct fed by a pump from a natural source of water, and run waterwheels over the aqueduct; but when I make an artificial reservoir and try the same thing, it doesn't work and I don't know why.
The key element seems to be staying as close as possible to natural water sources/sinks.