I appreciate the input and I say this for what it is worth but when I was in graduate school (fisheries major) I learned in limnology about the density factor of water. It reaches its maximum density at 39F. Above that warmer water will be on the surface. Once the water reaches 39F it will destratify and mix. Now that matches what you have all said B UT....
One of the other things we learned and did on field trips was to study the effects of things like minerals, tannic acid, algae, plankton dirt particles, etc on this characteristic. In Louisiana, where I went to school, there was a lot of tannic acid and dirt particles in the water which effected the water density. Most waters there turned over at a higher temperature (around 60F) depending on wind and rain. I guess that just stuck with me and I must have assumed that it would be the same here. Our water here would have different characteristics than Louisiana for sure. Thanks for the 'waking me up' to this.