Ian. That is normal. Valley really does not have any mayflies that hatch this time of year...let alone the rest of the year. Maybe some olives, and sulphurs, but they are few and far between.
I also stopped by Valley yesterday, no rod, but rather to just see what was going on. Took the temp above the Wilson Bridge, at 11:30am and it was 61-62F. Very nice and cool, but extremely low. Kinda normal flows for this time of year. I did observe a few fish riding very close to the surface actively feeding, not many, but a few.
Personally I rarely fish Valley during the summer, except for maybe a year with plenty of water, or air temps like had yesterday or today. Most of the time here in SE PA, I feel the fish are too stressed on that water to justify an angling outing.