As the weather turns cooler, lakes begin to “turn over” – a process that occurs when water on a lake’s surface is cooled to a temperature that is colder than the deeper water layers. Because the cooler water is denser, it sinks and mixes with the bottom layer, causing water on the bottom layer to also rise in part to the top.
“Lake turnover isn’t something you can see with naked eye,” explained Dave Robson, Forest Preserve natural resource management supervisor. “You would have to take temperature readings to show that it is occurring.”
But it does happen in a number of Forest Preserve lakes, Robson said, including Monee Reservoir, Whalon Lake and Lake Chaminwood.
The mixing of these lake layers increases oxygen by exposing a greater proportion of water to the atmosphere.
According to Robson, the rate of the fall turnover is influenced by several factors like the strength of the thermal stratification (the change in the temperature at different depths in the lake), lake size and depth, and wind conditions, but the turnover process usually lasts only a matter of days, sometimes even hours.
“Water is most dense at a temperature of about 39 degrees, so the process is over when the air temperature allows the water to cool to around that mark or a little above,” he said. “The fact that water below 39 degrees is less dense than water at 39 degrees is why ice floats.”
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