Per Sasquatch's PS, this is not a minor observation. The gauge is only a measured one-fifth of the way down the roughly 3.5 mi DH Area and there are plenty of solar radiation, wide shallow stretches, and pools that follow.
As I believe you know via your question, Troutbert, fish do not respond to thermal averages; they respond to thermal maxima and minima. They also respond to what are, in effect, degree days or other time-based thermal units. Warm temperature caused mortality can be chronic or acute, and the chronic occurs at temps that are frequently viewed as being sub-lethal by laymen.
And when fish are in thermally stressful conditions, or in thermal refugia, they are more vulnerable to predation, such as by great blue herons, which are effective predators in daylight and at night.