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If you're return line surfaces at a point where the waterfall begins you might consider installing a flexible return pipe. You can attach (same size) snake it down the spillway (or stream). It will enclose you're filtered water and act as a return, hence you have by-passed super cooling waterfall by enclosing the waterfall for winter. As Dick explained a cover will make all the difference in the world for keeping in heat.
Another idea, air from the bottom of the pond will help circulate water, keep the surface from freezing over and help with the earths 55 degree ambient temp (if you're at least five feet deep, seven feet is better) Air pump in an enclosed insulated box if outside or indoors, air also helps with some additional heat. A small electric spa heater will do the job for you for keeping water warm enough if you follow these other suggestions.
How do I know, live in Spokane, Anderson in Hayden Lake, both of us deal with harsh cold winters and have been through the routine, it works.
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