Lake water temperatures vary both with depth and time of year.
Seasonal changes in lakes and reservoirs
Temperature is also important in lakes and reservoirs. It is related to the dissolved-oxygen concentration in water, which is very important to all aquatic life. Many lakes experience a "turning" of its water layers when the seasons change. In summer, the top of the lake becomes warmer than the lower layers. You've probably noticed this when swimming in a lake in summer - your shoulders feel like they're in a warm bath while your feet are chilled. Since warm water is less dense that colder water, it stays on top of the lake surface. But, in winter some lake surfaces can get very cold. When this happens, the surface water becomes more dense than the deeper water with a more constant year-round temperature (which is now warmer than the surface), and the lake "turns", when the colder surface water sinks to the lake bottom.