Why do freshwater lakes usually not freeze solid from top to bottom?

Freshwater lakes typically do not freeze solid from top to bottom due to the unique density properties of water.1

While most substances become denser as they cool, water reaches its maximum density at approximately 4 °C (39.2 °F). As surface water cools toward this temperature, it becomes denser and sinks, causing the lake to mix. However, once the water cools below 4 °C, it begins to expand and becomes less dense. This colder, lighter water remains at the surface, where it eventually reaches the freezing point (0 °C) and forms a layer of ice.213

Because ice is less dense than liquid water, it floats. This surface layer of ice acts as an insulating barrier, protecting the warmer, denser water beneath it from the extreme cold of the air above. This process is vital for aquatic life, as it prevents deep bodies of water from freezing solid and allows organisms to survive in the liquid water below the ice cover.142