How do they know? I thought salmon always return to the same river, so a river no salmon come from won't get any returning, but I guess a certain percent are adventurous?

If all salmon returned to the same river then there would be only one river with Salmon spawning. Maybe they do have perfect memory, but a certain percentage of them get carried to other rivers by birds of prey who want to have Salmon in THEIR river

Most salmon do, but a small percentage always stray. If you think about it, it is kinda an obviously necessary behavior given that many current salmon habitats were not present during the last ice age.

The river wasn't entirely inaccessible to salmon, the dams just prevented access to the upper lakes and river segments.