Why Does a Lake Freeze from the Surface Down?

In winter, cold air cools the lake surface first. Most liquids become denser as they cool, suggesting that cold water should sink. Fresh water is unusual: it reaches its greatest density at about 4°C. As surface water cools towards 4°C in autumn and early winter, it sinks and the lake mixes. Once much of the lake is near 4°C, water cooled further to 3°C or 2°C becomes less dense. It remains above the deeper water and eventually freezes near 0°C.

The crystal structure of ice is more open than that of liquid water, so the same mass occupies more volume and has a lower density. Newly formed ice therefore floats. The ice layer also resists heat flow, slowing the loss of heat from the water below to the colder air. A lake can consequently have a frozen surface while deeper water remains liquid. The ice is not actively warming the lake; heat simply takes time to pass through it.

This is the basic account for a typical freshwater lake, not a rule detached from conditions. Wind mixes water, while depth, currents, dissolved salts and the rate of cooling can alter where and when ice forms. The central point is that surface freezing results from several connected facts: cooling begins above, water changes its density pattern near 4°C, ice floats, and the resulting layer changes subsequent heat transfer.

https://www.wtamu.edu/~cbaird/sq/2013/12/05/why-does-ice-form-on-the-top-of-a-lake/
https://courses.ems.psu.edu/earth111/node/842
https://seagrant.umn.edu/news-info/featured-stories/lake-river-ice-formation-classification