Is cold water natural?

[sea lamprey]

Regarding the concerns about warm water on the Delaware: I fish a good-sized freestone stream in Pennsylvania that by July heats up into the low 80s. It has wild browns and rainbows. And it has good hatches until late June. By July, it becomes a smallmouth fishery, but by mid-Septmber, the trout have become active once again, and so have the insects. My guess is that the fish find a deep hole, cold-water tributary, or spring to ride out the summer. Isn't that what happens on the Delaware during the summer? It's certainly deep enough and has enough tributaries. I remember reading in Joe Humphreys's book that trout will migrate long distances to find cold water in the summer. So while ice-cold releases in the heat of the summer might be good for fishing, is it necessary for the fish? Just curious, that's all.