Branch is another term for a stream, and virtually all of the good whiskey distilleries have one they use for their water. Typically the water's pleasant proper

Branch is another term for a stream, and virtually all of the good whiskey distilleries have one they use for their water. Typically the water's pleasant properties come from the geological formations it flows through, in particular limestone. So when you're watering back with branch water, you're using water of quality equivalent or even identical to that which to whiskey was watered back from cask strength with.