Density stratification describes the layers of water in a body of water with a layer of cold dense water at the bottom and, at temperatures above freezing, a warmer layer floating on top. When water begins to freeze, as water on the surface cools to 4 °C, 39 °F, it sinks. Eventually, the body of water will fill up with water at 4 °C and surface water will cool below 4 °C and freeze. Thus, in freezing conditions, water at depth will be warmer than the surface water in contact with the covering layer of ice.
The role of windEdit
During the spring, after the ice melts, the temperature of surface water rises, equalizing the density of surface water and deeper water. In those conditions wind and wave agitation mixes the water in a body of water, oxygenating the entire lake.