Why do some lakes and rivers freeze from the top down instead of all at once?

As a body of freshwater cools to 0°C, the surface reaches freezing first. The resulting ice floats because it is less dense than liquid water, unlike most substances. That floating layer blocks heat loss from below, so the deeper water often stays liquid even in severe cold. In deep lakes, this insulation can help fish and other organisms survive winter. The unusual density behavior of water near 4°C is one reason ice forms on top rather than the bottom.