E. Walter Le Roy wrote:

We have a 7 foot water wheel in a mountain stream and would like to
hook up an alternator, and light up the fish pond. Can anyone tell me
what RPM it needs to turn to run one headlamp?

Just offhand, with a motor idling at 1000 RPM, and the alternator
overdriven at least 3 to 1, I'd say you are at 3000 RPM