I'm in southern Canada, and if a pond is less than 36 inches below ground level, when winter comes, you hold a funeral....
A school I worked in had a large pond about 24 inches deep, dug into the ground. In the first year, a mild winter, I was called an alarmist when I said I was worried about the goldfish and cold, but the colleague in charge removed ice block fish for the three Springs after normal winters following. I started overwintering the fish in indoor tubs, which is about when I decided goldfish were not good indoor fish. It took, time, space and work.
The wild thing is your tiny pond may get super hot in summer. It is unfortunately not very practical, and more ornamental than suitable for fish. I don't know where you are in the USA, but here today it's -22 celsius today, and in July it can be +35.
Always remember that goldfish you buy can live more than 30 years if it's given a proper chance. That's a lot of water changes.
