What happens when a lake turns over 

By: Glynn Harris

In last weeks column, we talked about that “sweet spot” on our lakes, the thermocline. To refresh, the thermocline separates the warmer oxygen-rich layer of water separated from colder oxygen-deleted water below the thermocline. 

There is another feature on lakes in later summer that upsets the balance of the water above the thermocline and that below this line. It’s called “stratification” or as we know it best, when a lake turns over. 

I had to do some research to find out just what this phenomenon is, how it works, when it happens and the effect on the lake and its fishery. 

As autumn air temperatures decrease, the surface water of the lake begins to cool, causing the density of the water to increase. This denser, cooled water starts to sink, displacing the slightly warmer, less dense water below. This continuous sinking and displacement gradually deepens the thermocline, pushing it deeper and weakening the stratification barrier.