What is lake turnover? Simply put, it is the semi-annual exchange of surface and bottom water in the lake. This is caused by many factors, such as winds and weather patterns, but the most important factor is the change in water density as it changes temperature. Unlike most other substances, water is at its greatest density just before it freezes into a solid, which is why ice floats, and forms a cover over the lake. This wonderful property of water allows aquatic life to shelter deep in the lake, and survive the winter. Imagine if water froze from the bottom upwards - lakes would become solid blocks of ice, not only killing aquatic life, but also freezing shut the water intakes for city water supplies, and making shipping impossible.

The cycle begins in the spring, when the ice melts and water in the upper levels of the lake will begin to warm from right around freezing to match the temperature of water at the bottom, usually 39 degrees. As the water temperature changes, so does the density of the water, and the upper levels will sink to the bottom, 'turning over' the lake. Strong spring winds can mix the entire water column when the temperature is fairly consistent from top to bottom.

As time goes on, the sun will heat the water surface, and by late summer, the lake is stratified into layers where the water has different temperatures, density, and buoyancy. The wind can no longer mix the entire water column. Three distinct layers form, known as the epilimnion, the thermocline, and the hypolimnion. The epilimnion is the upper layer, usually no more than 20 feet deep, a zone of circulation, interaction with winds, and has a high dissolved oxygen content. The thermocline is a narrow layer where the temperature and oxygen content of the water rapidly declines, and the epilimnion is the bottom layer, cold and non-circulating, with very little dissolved oxygen. It does tend to contain high levels of dissolved sulphurous gases, such as hydrogen sulfide.

Fall, with colder, dryer air, allows the upper layer of the lake to lose heat, both through evaporation and a decrease in hours of daylight and sun intensity. As the water cools, it becomes denser and less buoyant. As the water temperature reaches 59 degrees, it will sink into the thermocline and epilimnion, mixing the waters and eliminating the stratification. As the water becomes more uniform, surface winds can once again affect the entire water column. Water from the lower levels will rise to replace the water that has been pushed away by the winds, causing an 'upwelling' of water from lower levels, and this action will work along with the change in water temperature and density, eventually 'turning over' the lake. Fall turnover, bringing water from the bottom to the surface, releases the sulfurous gases that will often be noticeable to local residents. Winter, of course, follows fall, eventually building up an ice layer over the water, starting the entire cycle over again.