It generally happens pretty quickly. Salmon seem to all have a "stray rate", where some percent will stray from their natal river on their spawning run. It make

It generally happens pretty quickly. Salmon seem to all have a "stray rate", where some percent will stray from their natal river on their spawning run. It makes sense, as it allows for colonization of new habitat, or to revive extinct populations. A lot of the current salmon habitat from Washington north was under glaciers not too long ago. So they have been doing this exact thing for millennia. Straying is partly why you can see salmon spawning above former dam sites, even shortly after removal. It's also why you will occasionally see salmon in a stream with no documented population (sockeye salmon on the oregon coast for example). As for how long it takes for a population to take hold I'm not sure. But I have read about salmon populations getting completely wiped out by landslides and recovering in 50 years or so, including the same adaptations (for example larger bodies for navigating a steep gorge) that the extinct population held. So potentially pretty quickly in the grand scheme of things.