I spent all of my life fishing Tennessee River impoundments that always had a decent flow and there was never a thermocline due to the moving water. Now that I have been here in Texas (DFW) for a year I am wondering if the lakes here generate a definite thermocline during the year? I've never had to deal with that before and is it something that comes into play here?

Many of the Texas lakes develop a low oxygen level layer in the summer. Many times it is so distinct that you can see it on the depthfiner.

This is on lake Whitney several years back. You can see fish relating to it

For sure it comes into play.

Here is a shot from Grapevine 4 weeks ago. Thermocline is set up strong at 25-26 feet. In a situation like that you will not find any life below 25 ft. As you can see fish will hold just above it but never below it.

if you have the settings on your unit dialed-in it's pretty easy to see.

Yes, most of the time 25 fow is the depth on most N TX lakes like Fork, Grapevine, etc. Deeper lakes vary like others have posted.