I just want to start off by saying this is another one of those things that I haven't seen mentioned or shown anywhere, so I don't know if this is an unknown or merely little-known design. However, it should prove useful for those worried about FPS and being able to STOP their generators.
People have created various ways to draw power from running water - usually rivers, or 'churning' it with the typical Dwarven Water Reactor design. The aquifer battery concept can produce a lot of power too, but it's limited in that it cannot be stopped once started and you're bound by building on the aquifer tiles. This limitation can prove annoying.
So I came up with something new. Simple, expandable and as easily controllable as the typical DWR - The Mechanical River.
How does it work? Easy! Draw water out from an aquifer with screwpumps, pump it under the wheels, then drain it back into the aquifer. The problem that may deter an interest in this idea is that the water wouldn't flow high enough to touch wheels along the latter half of the river due to the water draining into the absorbing row of aquifer tiles at the end. How do we fix this?
The water is drawn from the row of screwpumps (shown mid-pump) on the lower-left side and pumped around at a constant 7/7 level around the river and into the circled drainage tile.
This gives you a power generator you can expand as your needs grow with just a pickaxe some mechanisms and wood.
And above this, the heart of the river -
Although not in sync due to incomplete axles, all the wheels are indeed turning and generating power. The northern wheels will be expanded to fill complete rows, rather than the diagonal pattern shown, I just lack the wood to build them as fast as I'd like.
So now you have yet another option to enjoy when you need power to fill your dwarve's water source, or clense a cavern of all life with magma. You can build it straight, or wrap it around as I have done (due to space limitations) in any way you like. Enjoy!
(This project's admittedly half-finished, but I had enough done to show the proof-of-concept works and my current map doesn't have quite as many trees as I'd like)