It's not a genetic memory. They return to the place they were born. This is probably based on the "scent" of that place, and maybe other factors.

Some percentage either accidentally or deliberately go up a different river, which is how the species spreads. That's very likely who this story is about.

I think the point is if they "return to the place they are born" then why would they go back to the waterways freed up by destroying this damn. Clearly they have some heirachy in where they prefer to spawn and this place is at or near the top, or they would have opted to return to where they were born