My understanding of cold water stuff is this: Once water gets down below a certain temperature (iirc, it's around 34 - 35 degrees?) it actually becomes less dense & will rise to the surface (which is why ice floats) so this means that the 'warmer' water will settle in a layer at the bottom of the pond. You don't want to disturb this layering because then the fish won't have the warmer place to go down & hang out in. This is why you don't want aerators or pumps drawing from the bottom.
Running a waterfall & stream will definitely cause the water to get colder, but as long as you don't 'super cool' the water (dropping the temp very sharply & drastically as what could happen in high elevations or desert conditions where day/night temps swing widely) it should be OK.
Ice damming is definitely something to keep in mind, though, and if you either can't or don't want to have to deal with daily checking on things (at a bare minimum) in the cold weather, then shutting the falls & streams down is a better option.
Me, I love the look of a snowy/icy pond, so I deal with having to go out when it's cold & keep everything up & running. Fortunately here in NC we don't typically have temps cold enough for long enough to cause problems.