Do you maybe mean something like a fish ladder?

https://en.wikipedia.org/wiki/Fish_ladder

Or the cool "Salmon cannon" https://www.youtube.com/watch?v=2z3ZyGlqUkA

Am I seeing that right, they have a guy stand there, pick fish up, and load them in to a pressurised cannon?

I wad expecting something a bit more sophisticated/under the fish's control, even given the name!

They also have an automated system: https://www.youtube.com/watch?v=eGzdOpCisnQ

Can all fish jump? Feels like it may be something that salmon are pretty good at, but a gigantic fish may not.

Jumping is not always necessary. You can get an idea of possible designs for fish ladders from the photographs in the section "Types" in the article.

They could have, it just didn't happen that way. And more than likely it's to late for that species anyways. It could still save others though

When the dam was built, they didn’t know they were cutting off the fish from it’s only spawn point in the world (it was discovered later).

Still, there’s an argument to be made for building a bypass or fish ladder in every single new and existing dam regardless of what the current expected impact is.

I have issues with comprehending how that wasn't factored in from the start.

Then I remind myself most of humanity history is characterized by a marked tendency toward anthropocentrism, and I cease being surprised.

What surprises me now even more was that there were designs for fish ladders even as far back as the 19th century.

Studying fish habits when building dams is an ancient practice.

My guess: a political, hasty, rush to success to curry favor and clout drove bad decisions regarding a massive government project. Is that not the way of public bureaucracy, universally?