Why doesn’t water freeze under a thick layer of ice?

Ice has poor thermal conductivity, therefore, at negative air temperatures outside, under a thick layer of ice, it will always be relatively warm, at least the temperature will be above zero and the water will not freeze to the very bottom. It turns out that a thick layer of ice protects the reservoir from the cold, like a blanket. The colder the winter, the thicker the layer of ice that is needed to create the required thermal insulation.