IANAS but my understanding is they keep going upstream - while there's current to push against them - as an instinctual response. I believe water temperature also plays a role.

Salmon hatcheries also artificially boost the quantity of salmon in the stream.

If a salmon hatchery released salmon at the base of a dam, when the fish return and the dam was now gone, they'd just keep going.

However, there's more to it than this, because dammed rivers lacking salmon hatcheries have seen salmon runs start once the dams are removed.

I don't think the old adage that salmon will only return to their original spawning grounds is the whole story.