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

Most of them go back to their birthplace, but some end up elsewhere.

If that's a "deliberate" evolutionary strategy or just that 100% navigation success doesn't happen is unknowable.

Might just be some kind of salinity thing. Upstream -> Less minerals dissolved