But what about at the start when they are in the ocean and water isn't flowing from anywhere in particular?

+ How do they end up in one particular river and not another?

That part is not really known. Various things have been suspected like memory of magnetic fields, salinity, temperature patterns, odors, etc. Basically they may be memorizing those on the way out and end up coming back to the same shoreline. From there it’s following upstream water pressure (which is how salmon ladders induce them to follow the ladder).

Do rivers have a smell? Animals have a keen sense of smell and the volume of rivers is enormous. Seems like a random-walk sniffing for rivers would be effective.