What a tremendous undertaking. You are doing a fabulous job. That rocky cliff or outcropping is just amazing. I, too, am trying to figure out how best to winter the koi in my mud ponds. I have been putting a pool cover on one end, and I know it does nothing but freeze in place. I'm of the shallow bubbler school of thought. I want to keep those thermal layers. We also had a very long and severe winter here in the north east. I hope that was the worst one for a while. Continued best of luck to you and your wonderful pond.