Lake Turnover – A Fall Classic

By David Ernes – Lake Management Committee

In the fall, the kids go back to school, football returns, and we wonder when the Browns will win again. Another sign of fall is the lake turnover. It is a normal occurrence for a lake like ours that is fairly deep. The lake, during the summer, is much cooler at the bottom than at the top, where it is heated by the sun. The cooler water is more dense, and stays at the bottom. Because of this stratification or “layering”, the normal action of the wind does not mix the layers. Think of the lake as a black and tan beer – the more dense pale ale at the bottom and the less dense stout at the top. The layers tend to stay separated even as you drink it.

Now, as we go into fall, the temperature of the surface water decreases, and the density increases. It slowly starts to approach the density and temperature at the bottom. As the temperature difference between the layers decreases, the lake is more easily mixed by wind. This mixing takes place often around the last two weeks of October. A number of things happen – the lake is no longer stratified, and the materials at the bottom of the lake can now reach the surface. Since the nutrient levels at the bottom of the lake are higher than at the surface, this can sometimes (but not always) result in an algae bloom. You may also notice a slight sulfur odor as other materials at the bottom make it to the top. This usually ends after a few days.

After the turnover, the surface water continues to cool. Once it reaches 39 degrees, something very interesting happens – the density of water starts to decreases. This is why ice floats on water and deeper lakes will likely never completely freeze. The ice and colder water forms a blanket on the surface of the lake, so the main body of water never drops below 39 degrees. This is beneficial, as the fish do not end up as frozen fish sticks. A similar turnover occurs in the spring as the surface warms.

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