Nature creates a perfect circle of ice

We went for a brisk walk this morning in the brisk fall weather, and on Hunsacker Creek the water action on the freezing surface made a ice disk...cool huh?! I got my video camera out to share this wonder of nature. Ice is sooooo cool!! (I guess you can tell that I've lived in Alaska for too many years to be so excited about ice!!)
So all you math fanatics.....why the perfect circle? I know....do you????