In most of Nebraska, deep lakes will tend to stratify or form layers, especially during summer. This happens because the density of water changes as its temperature changes. During the summer, a temperature difference is established between the upper water and lower water in a lake. The upper layer of water, called the epilimnion, is warm. This layer is about as deep as the sun can penetrate. The deep layer of water, called the hypoliminion is made of cooler water.