One of my favorite vacation places is Desoto Caverns in Childersburg, AL. It is a family place, filled with all kinds of things to do for kids and adults. One of those things is to "pan for gems" in the water-filled trough that features running water to help separate the sand from the gems.
Part of that set-up is a real old-fashioned water wheel that powers the operation. Here is a picture of that water wheel doing its thing with the water flow also heading down the troughs to the gem panning area.
Click on the image to make it larger.