I believe that Bonneville Dam has 24 hour counts. At night, they use a video camera to record the fish migrations and based their counts on that. It's probably easier than visual counts because they can re-wind the video to make sure their ID is accurate. The other dams don't all use video and may collect the migration data during daylight hours only.

However, your basic point about delayed migration is right on the mark. When the water temperatures get too high on the mainstem Columbia (over about 65 F), the fish stop moving. They often will pull into the tributaries where the water is cooler. Drano Lake, Deschutes River, and White Salmon River are known for their summer steelhead fishing. Many of the fish caught in the lower end of these rivers are upriver fish that have stopped to cool off. They will resume their upriver migration when the mainstem cools down. Alot of anglers know this and take advantage of it by targeting Drano and White Salmon rivers for steelhead during the warm summer months.