There are three different layers. The top layer of water is heated by the sun and stays warm. The middle layer is cooler than the top layer and the bottom layer is the coldest. Because of the warmer waters and more plentiful food supply, almost all lake creatures spend the summer months in the upper layer. During the fall there is less sunlight to heat lake water during the day.
Lakes can lose more heat at night and wind can mix up the water layers.
When all of this happens, the water in lakes can become close to the same temperature at all levels. This is called fall overturn . The top layer is the coldest. When the surface of the lake freezes, the water underneath is protected from wind and heat loss. About 80 percent of all the lake water in the world can be found in 40 large lakes and 70 percent of all the lake water in the world is found in North America, Africa, and Asia.