Well the warming starts in the headwaters, even though there are good wild trout streams in the headwaters. Even brookie streams, But there is Leaser Lake to warm it and I assume another dam from what Mike said. Some of the tribs are even too warm for trout.
I'd bet the original forests were removed in the early 1700's, that's what happened in the Perkiomen Watershed. The biggest difference there is the Perky has cold water tributaries and is smaller, even though the flood plain is very wide for the Perkys size. The Lehigh River used to pass through South Mountain at Leibert Gap as opposed to where it empties into the Big 'D' now. The ice age changed that. Not sure exactly how that came about, but that's the history, and it explains the name Perkiomen too (place where cranberries grow).