Don't confuse frost line with how much of your pond freezes. Frost line is the depth at which groundwater freezes. There are a lot of factors that keep your pond from freezing to the bottom (warmer days, sun, etc.), but once the ground starts to freeze, it will keep going deeper and it most likely will stay that way all winter. It takes a long time for the ground to defrost. You may have early warm spring days with moderate temperatures, but just below the surface your ground will still be frozen. I am in PA Zone 6 and my frost depth for my soil is 44 inches, however I have an above ground fishpond that is 36 inches and only the first few inches on top ever freezes. After a couple of cold weeks where the temps were single digits at night and teens/20's/30's during the day, we had 4 or 5 days in the 40's/50's last week. I still couldn't even drive a wood stake in the ground more than 1/4" before the ground was like a rock, but my pond was ice-free.
