I'd venture to say that a few friends and I probably fish various stretches of this river as much or more than anyone. As I mentioned in a post a few months ago, we caught trout well into November last year which, I believe, means they "held over". I have no doubt that trout hold over in the Brandywine and it's West Branch in very good numbers. I'm not familiar with the East Branch, but plan to give it a shot in September or after. For some reason, the East Branch doesn't seem all that appealing.
I haven't fished the main stem in Chadds Ford, but keep hearing stories of nice trout being picked up behind the River Museum in the area cited by TDB in his post. I should get out there since it's about five minutes from my house. LOL. While Pocopson is stocked a mile or two upstream from the museum, the "Club" also stocks the West Branch about seven miles or so up in the West Branch. The State of Delaware stocks a few small tributaries downstream. I have little doubt that these fish make their way into the main river and hold over there as well.
Buck Run, with a decent population of streambred browns, enters the West Branch near Mortonville and these fish also venture out into the bigger water. My point is that over the years, mostly through stocking, plenty of fish have been put into this stream which, by the way, is beautiful for long stretches and clean and crystal clear with aquatic life beyond any expectations. Couple this with the fact that trout, especially the dreaded rainbow (God forbid), are known to move up and downstream in excess of ten miles leads me to believe that there are more of them in the Brandywine and its West Branch than we might think.
I also believe that the survival instincts of these fish would lead them to seek out colder water near springs or heavily shaded areas than simply rollover and die in the warmer stretches. Where are all of these dead trout anyway?
Mike, since you started this, why don't we a schedule a time to sample sections of these streams to see what's there? I would be happy to help in anyway.