Though we've had a cooling trend recently the water temps on some lakes are still at lethal levels and mortality rates will still be high. I'm not telling anyone what to do, I just want everyone to understand that the lakes you think will be cooler might actually take much longer to cool off. In the end it's your right to choose to fish while the water is hot and when mortality can be high. If you must get out on the water the lakes that are shallow will cool faster than lakes that are deep.

It doesn't make sense, a deep lake should be cooler – right?

To a certain extent that is true, there will be water within the lake that is cooler. This cooler water will be at and below the thermocline and does not mix with the surface water (Epilimnion). The deeper lakes that develop a thermocline will cool slower because only the warm water above the thermocline will mix as the surface water cools. When there is an extended period of hot water the thermocline is pushed deeper and there is more hot water above that will need to cool. As the nights make the surface water cooler it will sink through the warm water below. This criculation happens above the thermocline and slowly the thermocline will begin to rise in the water column. For this reason lakes that don't develop a thermocline will mix faster because the cool surface water will push to the bottom and drive that warm water to the top where it can be cooled.

Here's a diagram that illustrates how this mixing happens:

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Just something to think about.