Why lakes freeze from the top

When water freezes, its density decreases, so ice floats. In freshwater lakes, water is densest at about 4°C: water cooled to that temperature sinks, while colder water stays near the surface and freezes first. The ice cover slows heat loss, so many lakes do not freeze to the bottom and aquatic organisms can survive the winter. This is not guaranteed in every shallow or severely frozen water body.