I have an infra-red thermometer. I tended to lose glass-tube ones. I put them in the water, made a few casts while waiting for the reading, caught a fish or otherwise spaced out, and forgot the thermometer. I'd like to say that I only did this once, but ...
I think infra-red thermometers are good, because you may use them more often when they work faster. stream temps can change sometimes more abruptly than I would guess.
I just fished a small brookie stream that had great heat-wave water temps: 61 in august, nice. after 3/4 mi up a mountain, the channel was there, but suddenly the water was gone. last bit of water was 58. the water stayed underground below a perfectly normal but bone dry stream bed of ledges and pools. never made it to more water, went up another 1/4-1/3 mi.
But I looked at google earth, in higher flows there's clearly water running right through the 1/4-1/3 mi section that is dry now.
I assume that the fish population is lower that intermittent stretch versus downstream even when there's water all the way.
So let's say I'd fished the stream in may. maybe by taking a lot of water temps, Id have noticed that the fishing fell off and the temp jumped in that section?