At the top of the waterfall, the water has a certain amount of energy. Namely, it's potential energy, given by
[tex]E_{p}=mgh[/tex]
Where [tex]E_{p}[/tex]=potential energy
m=mass
g=gravitational acceleration, ~[tex]-9.81m/s^{2}[/tex] on Earth's surface,
and h = the height above the surface, or above the "reference point".
So what happens when the water falls onto the wheel? Where does the water's energy go? Does the water do
WORK on the wheel? How much energy would it take to raise the water back to it's original height? Do you see the problem?
Hint:conservation of energy! In order to raise an apple above a table so that it can exert 10N of force on the table when it falls, you must use 10N of energy raising the apple to that point if you pick it up from the table.
If you don't have a textbook, what exactly have you gone over in class? review your notes, I mean, you had to have learned something that you can apply here.