I'll stick to what I see in my backyard, my readings & thermometer and you can stick to being an armchair expert 3000 miles away.
As I suspect you fellows know, temperature readings can vary by location on the same stream. I say that only because there are differences among the three strams, upper Perkiomen (Class A section, less than Class A wild trout section), Pine, and Bieber. The Class A section of the Perk has some pretty darn low gradient in places and can be a bit sluggish, which under low flow conditions can boost water temps. Farther down the gradient picks up and becomes more consistent and steeper, so it would be no surprise if it is cooler down there. Once it gets to Hereford the gradient declines again, but because it is now shaded much better than 30 yrs ago, the abundance of wild trout down to the stocked stretch seems to have picked up.
Pine starts as a deeply shaded, wooded wetland with low gradient that quickly improves. It is cold in the upper part of the Class A stretch all of the way down to near Lobachsville, where it starts to warm up and loses gradient. It probably gets somewhat warm at times just below Lobachsville to its confluence with Bieber.
Bieber starts out warm in the headwaters near Dryville (creek might have been dry there when I passed over it yesterday) and then gets cooler between Dryville and Rt 12/Pricetown Rd. It gets much better below there as the gradient really picks up, the stream enters and passes through a hollow, and it picks up some small springs and trib(s). Exiting the hollow it loses gradient but maintains a fair amount of shade. It gets warmer, but still supports a Class A population and carries that all of the way to the Pine confluence. Would not be surprised if it warms into some stressful temps in the very lower part of the Class A segment at times, but it has improved over the years sine the upper half of the Class A stretch was established following a 1988 survey, but the lower part did not become Class A until around 2015 give or take, and that was because of temperature effects for the most part.
Without any attempt at being flippant, the beauty of all of this is that fish can swim. This allows them to move upstream to cooler temps (usually, but w/ exceptions in some waters as noted above), or to trib mouths, upwellings beneath trib mouths, or to the rare, larger “mid-channel” upwellings. As I said previously, I only ran into or saw a pic of a sizable, multi-fish (wild trout) accommodating, mid-channel upwellings or springs twice in my career on hundreds of trout stream sections. Of the two, one was occupied by wild browns; the second was not being utilized by the wild brooks because the major upwelling, which for all intents and purposes was a mid-channel spring, and other near-by springs were keeping the entire downstream channel cold. That spring (upwelling) was the upper limit of the Class A section where the vast majority of the ST lived year-around.
On another occasion (ST) and possibly a second (BT), I was fortunate to be at the right place at the right time to have discovered that most fish in a localized population had moved, presumably upstream, in a matter of a day or a few days and, of course, the 1977 tagging study in Penns showed considerable movement of at least a segment of the population in an upstream direction. In contrast, however, I have seen wild BT in another warming Class A stream apparently just hold position while in obvious stress. Perhaps they moved at a later date. I suspect that Penns occurrrence could have easily been temperature related given that they moved from a known marginal temp area to a known colder area.
I don’t presently buy into the idea that cold upwellings are all that important as temperature refuges in much of Pa only because I don’t think they are very common or generally very large, large enough or frequent enough to accommodate enough fish to be important at the population level. They might be important in keeping cold sections uniformly cold, such as at base flow, but not in providing well defined refuges for tight congregations of fish in warm or warming stretches. Perhaps they are in other geological formations. I am not saying that I have not seen an occasional single trout in a cold seep coming out of an embankment along a warmwater river or stream, such as the lower Delaware or middle Schuylkill where it is intersected by some limestone geology, but the occasional refuge for a single fish is not what I am speaking about when speaking large upwellings.
Clearly, fish have various strategies to adapt to warming water conditions. In my view trout movement is a more effective and more common means for mass escapement from warming waters as long as it is done before the temperature stress and penalties for such energy expenditure in warming water are too great.