These are simply thoughts of a non intrusive method to capture the power of water flowing downhill...Please take a moment to consider the possibility of such a scalable engine powered by water. First go to the following link and view the concept of using buoyancy to lift weight. Refer to the last drawing at the end of the article. I say scalable because ships are lifted through the Panama Canal using the same principal.
http://debateusa.com/debate/viewtopic.php?t=1337
Now go to the below site and view an animation of the old John Deere two cylinder engine and how it functions. If you turn the animation clockwise on quarter turn and connect the two concepts in your mind you have a scaleable water engine. http://my.voyager.net/~vinsond/jd_animation.html
Please note the original drawling describes a storage tank however for a large scale power source a stream could be routed through step tanks similar to the Panama canal to achieve the same action. The exhaust valve would evacuate (cause to flush) the water to the next tank.
An after thought I had considered instead of the buoyancy pushing they could be tethered to a pulley at the bottom of the tank and transfer the power through linkage connected to relative levers to propel the crankshaft connecting rods.