So how do they find the river outlet into the ocean? There surely is some bird compass thing involved. I am only half joking when I write that Venus guides them.

That nature works at all is astonishing.

It's all chemoception, the same as with single cell organisms. They swim towards a saline gradient ( which they can taste, for sure ) and follow it up into fresher and fresher water.

Word gets around? Animals probably have way better communication than we think. One crab says to a friendly eel "hey dont tell those damn Salmon but this estuary is good again" and before you know it, everyone's favourite restaurant is booked out till March.