I don’t think a short run of cooler temps will suddenly shift most of the fish back to the lake, though it may start things off. I’m not too familiar with that fishery, but fish migrating to spawn don’t respond that way. They filter in, then out, of the river over periods of weeks. I would think a super quick response in this type of situation would not be adaptive–the situation they are trying to avoid in the impoundment will persist for weeks.
I had an unremarkable river striper year, and summer was the unremarkablest portion of it. Here’s a bad self-portrait.

G