Def appears to be! Real cluttered return at/below that and you don't see any fishy looking returns below that line. Seems to correspond directly to the drop off into deeper water.
Water temp? What part of the country were you fishing when you saved these sonar images?
Could be, and I'd treat it as such (I wouldn't be fishing below 15ft)
Could also be a layer of debris/silt from all all the rain we've been having. Been seeing these silt layers in North Texas at a few lakes. It's a little early to be seeing a thermocline in N. Texas (still having night time temps below 80 degrees), but certainly could be starting to set up.
Seems pretty shallow for thermocline. Is this a stagnant lake? When it devolpes here, it does not get that shallow until the temp gets higher and water isn't flowing