At times during the late winter, one can encounter slush beneath the snow. This can be quite alarming, but is usually not an indicator of thin ice. There can be slush ice on 20″ of good ice. Having said that, one has to always be aware of thin ice, or very weak ice. If you think there is not 5″ of good ice, then simply don’t trust it.
Typically, slush ice is formed because a burden of snow weighs the lake ice down, thus forcing water up through any cracks or crevices as the ice layer tries to sink. Because the snow is an insulator, this water forms a layer of slush at the snow-ice interface. And when the ambient temperature does not provide enough cooling (usually late February or March), there is then not enough heat extraction to allow it to transform to ice. (The heat of fusion is 80 cal/gm, which means that a considerable amount of energy is involved in changing a gm of water to a gm of ice at 0 degC)
Melting snow, or rain, can also contribute to the water content of the slush layer. But simply stated, it will remain slush until conditions are such that sufficient heat can be extracted to allow it to freeze. If it is able to freeze, it usually freezes from the top of the slush layer down, and creates what some call “snow” ice, which, if not frozen all through, can cause a sandwich of slush between the snow (white )ice and the lake (black) ice.
See the following articles for more information:
Chandos Lake Science 2 