When they're running its always possible to catch them.
However, conditions may dictate your chances on doing it.
If you know a system well enough there is always opportunity but for many, with the reduced chance, it is too far a drive to go play.
As far as swimming hundreds of miles...
Not every run or species of salmon has the same capability.
Different runs have different DNA.
Their genes determine that ability.
Not every horse can run in the Kentucky derby and have a reasonable chance at winning.
Very few of us will become elite athletes (though many think they can).
High coloured water:
reduces the productive areas to fish.
Moves the fish to areas most don't know to fish.
Creates Hazards for the inexperienced.
You don't have stop fishing but realize your chances are diminished and the danger is increased.
Go fish, learn something.
But most importantly: be safe, it's just a fish.