Lake Superior can see temperature differences of 20-40 degrees between the upwind and downwind shores. While the effect is less pronounced on Sak, I have seen the thermocline move as much as 20 ft or more on the face of the dam in a few hours. Going back next week with a vengeance. Did see some pulled out, but the general consensus has been one or two per trip. Your not quite correct.
Turnover is caused by cooler water sinking into the warmer water below it.
On Sak in my experience, when the water surface temp cools 10 deg. This is easily seen by watching the thermocline. When the thermocline starts to rise as it has in the river at New Town there has been some turnover. Remember we had very hot weather followed by a week or so of cold weather a little while back. For example I was fishing DL and the surface water temperature was 86 Deg.