Why do ponds and lakes freeze from the top down?

By the end of summer, the coldest and heaviest water is found at the bottom of a lake or pond, with layers of successively warmer water above it. When the air temperature falls below freezing, the surface water turns to ice, and if the subfreezing temperatures persist, the layer deepens and thickens. As the ice floats in the water, it acts as an effective buffer between the deeper water and the cold air. Anchor ice forms on the bottoms of streams and lakes when the layers of water are uniformly cold, but a rise in temperature will cause it to float to the surface.