The last couple of Winters were pretty mild. This year turned out to be more typical, with cold spells lasting 3-4 weeks at-a-time. I decided to try and leave the pump running (without any water heating device). At first, I wasnt too concerned since the water kept running even on nights when the temperature dropped to 5 degrees Fahrenheit. And, everything looked so nice...
After a couple more cold nights, the water level started to drop. I'd add more water during the day and by morning the level would be down -- a couple times going below the pump intake -- leaving barely enough to circulate. I looked for places where an ice buildup could cause water to flow out of the pond but couldnt find anything. A closer look revealed the problem. It's the ice itself... duh! The water freezes on top of the ice and the water level drops!
Doves... at any given moment, we could usually count over 20 doves hanging out at the pond. Other birds and squirrels too, but the doves pretty much owned the place. They'd be taking baths, even on the coldest days (as long as the Sun was out). Incidentally, the water is running there... mostly under the ice!
This was after an 8" snowfall; snow doesnt seem to pose any problem. It's the ice. The temporary solution to the "ice problem" was for me to go out in the morning with a bucket of warm water. I'd pour it over the falls area, melting the ice and quickly bringing up the water level. Usually 1 or 2 buckets was enough.
More snow, and about 8" of ice now. Experience taught me to keep a path clear from the falls to the pump (bucket brigade). This helped to prevent water loss. The general problem was this: water runs down the falls and under the ice. Something (e.g., a leaf) blocks the waterway causing the water to flow over top of the ice and freeze. The ice level would rise, the water level would drop.
One last shot of Winter. When it wasnt too cold (20 degrees or more at night), there wasnt much to do. The pump filter required cleaning only monthly since nothing was growing in the water. I got filter cleaning down to 2 minutes, anything more and my hands would have become frozen. Next year, I need to think again about leaving the pump running all Winter. I need a better plan; I do not want to do the bucket thing again!!