Many of our Illinois lakes and reservoirs are deep enough to stratify, or form “layers” of water with different temperatures. Such thermal stratification occurs because of the large differences in density (weight) between warm and cold waters. Density depends on temperature: water is most dense (heaviest) at about 39°F, and less dense (lighter) at temperatures warmer and colder than 3 9°F

The Stratification Process

In the fall, chilly air temperatures cool the lake’s surface. As the surface water cools, it becomes more dense and sinks to the bottom. Eventually the entire lake reaches about 39°F (4°C). As the surface water cools even more, it becomes less dense and “floats” on top of the denser 39°F water, forming ice at 32°F (0°C). The lake water below the ice remains near 39°F. This situation is referred to as winter stratification. Winter stratification remains stable because the ice cover prevents wind from mixing the water.
Come spring, the ice melts and the surface water begins to warm above 32°F. The increasing density of the warming water along with wind action cause this surface water to sink and mix with the deeper water—a process called spring turnover. During this time period, most of the lake water is at the same temperature, and surface and bottom waters mix freely. Lakes with a small surface area, especially if protected from the wind, typically completely mix for only a brief time in the spring—usually just a few days. In comparison, large lakes often circulate for weeks.
As the sun continues to warm the lake surface through late spring and early summer, the temperature differences increase between the surface and deeper waters. In lake areas deeper than about 10 to 12 feet, the temperature differences eventually create a physical force strong enough to resist the wind’s mixing forces (it only takes a difference of a few degrees Fahrenheit to prevent mixing). The lake now stratifies into three layers of water—a situation termed summer stratification. The upper layer is a warm (lighter), well-mixed zone called the epilimnion. Below this is a transitional zone where temperatures rapidly change called the metalimnion. The thermocline is a horizontal plane within the metalimnion through the point of greatest water temperature change. The metalimnion is very resistant to wind mixing. Beneath the metalimnion and extending to the lake bottom is the colder (heavier), usually dark, and relatively undisturbed hypolimnion.

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