They achieved this by putting the ladder on the opposite side of the channel from the locks.

So the think with salmon is that they orient by sound. If they don’t know where to go next they head toward the sound of running water - the next rapids upstream. To get them to traverse a dam you have to make waterfall noises in the correct spot and not make them in the wrong spot. I believe I’ve read of people using this to trick salmon into going through locks, but the ladders seem to be simpler.