A lake might experience what scientists call “overturning” twice a year — in the fall and spring. In the fall, the surface of a lake begins to cool down. Cool water is denser than warmer water — so you end up with water on the lake’s surface that’s more dense than water below. The denser surface water sinks. It’s replaced by warmer, less dense water, which — once it’s on the surface — proceeds to lose its heat, become more dense and sink too.
This cooling and sinking of water doesn’t go on indefinitely. The entire lake can reach a temperature of “maximum density” — about four degrees centigrade — 39 degrees Fahrenheit — not far above freezing. At this temperature, the lake stops turning over. The surface of the lake may then cool further and start to freeze.
In spring, the lake turns over again. Surface ice melts — and the surface water becomes more dense again, as it once more approaches 39 degrees Fahrenheit — the temperature of maximum density. The surface water sinks, and the process of overturning proceeds, moving from shallow areas near the shore to the center of the lake. It’s only when warmer less dense water floats on top of cooler, denser water that the lake becomes thermally stable once again.
The temperature of a substance determines its density — how tightly individual molecules are packed together. And the more dense a certain volume of a substance is, the heavier it will be. This is true of all liquids. But, unlike most other substances, water’s greatest density is not at its freezing point — 0 degrees Celsius — but 4 degrees higher. This gives it some very unique properties. It’s why ice floats on water instead of sinking. It’s also why water experiences overturning as outside temperatures go from above 4 degrees Celsius to below 0 degrees Celsius, or vice versa.
The heavier something is — or the more dense a certain volume of it is — the stronger Earth’s gravity pulls on it. In a lake, gravity constantly tries to keep the least dense water at the top and the most dense water at the bottom. When temperatures drop to 4 degrees Celsius, you have cold, more dense water at the top and warm, less dense water at the bottom. Gravity tries to reverse this distribution so that the most dense water is on the bottom and the least dense on top. Overturning occurs as long as there is some water still above 4 degrees Celsius. This less dense water continues rising to the top. Then as outside temperatures continue to fall, surface water becomes less dense again, so overturning stops.
In spring, when temperatures rise back to 4 degrees, the opposite happens. During overturning, a large part of the water in a lake circulates from top to bottom in just a few weeks. The Great Lakes are massive inland seas.
If you enjoyed this article, the following books may be of interest:
John R. Vallentyne, “The Algal Bowl: Lakes and Man” Dept. of the Environment Fisheries and Marine Service, 1980, pp. 37-52