Fans of waterwheels might find these articles interesting/useful:
http://www.backwoodshome.com/articles/behrens16.html
http://www.backwoodshome.com/articles/behrens17.html
http://www.backwoodshome.com/articles/behrens18.html
While undershot waterwheels are not the most efficient, they are very easy to build and one doesn't need a penstock to feed them water either. You do need to have a fast moving stream (i.e., average velocity greater than 4 FPS) in order to generate a significant amount of energy because the effective head created by this fast moving water is proportional to the square of this velocity. It should be noted that the British industrial revolution was initially started with these machines before steam engines came along.
I created and attached an Excel spreadsheet to help you waterwheel fan DIYs with such a project. :)
Cheers,
Bob B.