This is why ice floats. The other extreme is also true. As water warms from 39 degrees it becomes less dense and lighter as molecules speed up, ultimately breaking apart, changing to steam at 212 degrees. Reservoirs upstream from tailwaters warm and cool with the changing seasons. As the days lengthen, reservoirs warm, absorbing heat from the sun.
Wind blowing across the surface mixes the reservoir, bringing in life giving oxygen to all depths.
However, as spring and summer progress, surface temperatures continue to warm, water become lighter and less dense, while the deep water remain cold and heavy. The circulation pattern begins to change. It takes more and more wind energy to mix the reservoir from top to bottom because of differences in water temperature and density. At some point in early summer, the reservoir stratifies with warmer and lighter, less dense water, on the surface, and heavier, colder, denser water on the bottom. Biologists call the upper, mixing layer the epilimnion and the deep, non-mixing bottom layer the hypolimnion.