Jay is correct, my implication was that Cacoosing adds cold water that allows fish in that area and below to hold over better. I was not saying that the water temperature in any way affects herons.
I haven't been down there much this year, so I'll trust the temp readings that were given. I took plenty of readings last year.
#1: The waters of the Cacoosing and the Tully do not mix very well for at least several hundred yards. One side of the Tully will be significantly colder than the other. The cold side is shallow and flat except for right at the PaperMill. But yes, when the water gets into the mid-70's elsewhere, all of the cold areas are full of trout.
#2: Even if the effect doesn't last for more than several hundred yards, thats quite a thermal refuge that can hold a lot of fish. Those fish will spread out again come fall, and most will go down.
This, IMO, is why there are more holdovers below than above, which by the way, my observations mirror yours. That said, the fishing for this year's fish is just fine anywhere, until summer's water temps do them in, and its stocked in the fall too.
As far as holding water. Waterworks is quite frankly poor water, but its stocked heavy and thats all that matters anymore. Rebers is alright, refrigerator hole is alright, papermill is pretty good, etc. But its telling that only the name pools offer anything as far as holding water. And none of them hold a candle to what's below Gring's Mill, thats the best water on the stream. But of course, holding water is only part of the equation. Water temp, regulations, and where they stock can't be removed from a discussion on what areas fish the best at certain times.