Had my kayak out yesterday on my in-law's lake behind their house with my Lowrance Elite 4x-DSI on it and noticed a distinct line between 5-7 feet wherever there was deeper water than that. Is this a temperature thermocline showing up or something else? Pictures attached..

What type of lake? Was it pretty small? I ask, because what you are seeing is too shallow to be related to temperature (i.e., a thermocline). I suggest the clutter you are seeing is a thick plankton bloom. Ponds and small lakes can be very fertile with high plankton densities.

If that maxes out the depth of the lake, it might not have a thermocline.

Mermet springs is a dive facility. This will give you an idea of the drastic change once you hit a thermocline.

• Surface: 86°F | 15' - 25' Visibility
  
• 15 Foot: 82°F | 15' - 25' Visibility
  
• 20 Foot Platform: 81°F | 15' - 25' Visibility
  
• 45 Foot: 55°F | 20' - 30' Visibility
  
• 55 Foot (Thermocline): 51°F | 25' - 40' Visibility
  
• 85 Foot Platform: 42°F | 25'+ Visibility
  
• Bottom: 42°F | 30'+ Visibility
  

Yes, you can have a thermocline in a lake that has less than 25' deep water. I used to fish at a 75 acre lake frequently that the late summer thermocline was at the 10' level with a max depth of 22'. I could see it with my sonar and verified it with a remote temp sensor thermometer. The bottom temp was in the 50's with a surface temp in the 80's. The thermocline was the pattern that made catching quite easy.

I'm thinking that's what it is. We had some really heavy rain the two days previous from the remnants of Issac. The day after that picture the line wasn't there anymore. I'm thinking the water was mixing from the rain as the water temp about a month ago in this pond was around 82-83 degrees. And as you can see the surface temps the other day were around 78 or so. I think the rain cooled the surface down, the water mixed and yesterday it was mixed enough to not show the difference on my finder anymore. Yesterday the surface temp was around 80-81. The bite was off too as I only managed about 10 compared to 30 or so the previous day.