I once asked an older sawyer how the would start and stop a water powered mill, in case of some need such as service the blade. And I asked what kind of "clutch" was used to disengage the arbor.....
You can't imagine the look he gave me......
And he simply stated, you "stop the water......."
It seems that most mills that he was familiar with had some type of water way that could be opened and closed by inserting or removing some short planks in a chute.