First thing to do is to figure out how much head you have.
Identify the sites where you can:
- Pull the water out of the stream.
- Build your generator and put the water from it back in.
If you can't get a good measurement of the height difference, run a hose from one to the other, get all the bubbles out of it, and then (slowly so it doesn't break) turn off the water. Once it stops, measure the pressure at the "put it back in" site's height (or a known height above it). One PSI is equal to 27.7 inches of water column, a little over two feet of head.
Another way to get a quick estimate the head is to aim the nozzle straight up and see how high it shoots. But PSI and survey maps are more accurate.
Next you need to estimate how many gallons per second you can divert through your generator.
Finally you need to figure out a route for a pipe - how long it is and what sort of bends it has.
With those three numbers you can determine:
- How much power is in the water.
- What size penstock pipe to use.
- What type of turbine would work well.
- What efficiency to expect in extracting the power.
It sounds like you have a lot of head so you'll probably be able to use a Pelton or Turgo, or maybe a Banki/crossflow. Peltons and turgos are nice because they are efficient and their RPM is high, good for generator design. If you're in the right head/flow range for a Banki they're nice because they're easy to fabricate and self-cleaning, though slightly less efficient.