Can you elaborate a bit more? Is the trommel to be in the water, or is the power to be transmitted onto a land unit? What size do you have in mind? How portable are you looking for? What's the water flow and other characteristics where you wish to use this?
I had an idea years ago about using water flow to power a small charger that would keep a 12v water pump going all day. Didn't need a lot of water flow for what I had in mind, but also the water flow was very minimal. My final idea was to make a floating dam that could be extended and would harness the top 12" of flow across the stream. If you have ample water flow, this will not be an issue.