I think one of the biggest secrets to overwintering your pond is in the construction - your waterfall has to be built in such a way that it can form ice without the water being diverted out of the pond. Our waterfall is wide - almost three feet across - and in the coldest parts of winter it ices over completely, but the water continues to flow under the ice. The waterfall was also constructed to be concave - the sides are higher than the middle - so the water isn't going to get pushed out when ice forms. The pond will ice over completely, but the water continues to flow, meaning we don't have to worry about de-icers or keeping a hole in the ice. The only winter time concern we have is watching the water level. We have a special short hose that I keep in the house that we use to fill the pond when the water level gets low, but we have only had to use it twice in five winters.
We actually shut our pond down the first year. We really didn't think it was possible to keep it running. Because of the way our pond is constructed, turning off the pump drops the water level about six or eight inches, which left all of our marginal plants exposed to the elements. Every single one died. Big disappointment. So I started researching and brainstorming ways to protect them the next year - maybe I could use burlap? Or bales of hay? Or a combination of both. Suddenly it hit me that what the plants needed was to remain covered by water - or ice and snow - and to do accomplish that, the pond needed to stay running.
We prepared ourselves for every possible disaster that first year. We had the de-icer and auxiliary pump in place in case we needed them. We crossed our fingers and left the pump running. And it was fine. We had one of the coldest winters on record here in Chicago that year - had we known that was coming, I don't think we would have had the guts to try it, but we did and it worked fine and we were relieved and pleased to see all our plants come back that next spring. And the water was crystal clear and the fish were all healthy. It was an experiment that ended well.
Here's a picture of the pond from that winter:
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That was a long, cold winter. The waterfall is to the left in the photo. The water is flowing towards the top of the picture, over the negative edge into the rain exchange. The water is pumped back up to the bog where it feeds the waterfall. The bog will freeze over completely, too, but the water continues flowing. We enjoy seeing the interesting ice formations that are created each year and the lights on the waterfall are really pretty lighting up the ice at night.