Apparently, they are able to smell molecules of their "home" waters. My presumption would be that after allowing their magnetic receptors to lead them to the vicinity of their native streams, the smell receptors take over, detect familiar molecules, and gravitate toward the highest concentration of this familiar smell until they are in their home streams. In Valdez AK (home for 18 years) we saw return after return. Hatchery stocks and wild stocks from the same area returned without mistake every year. (Hatchery fry were marked by removal of the small adipose fin near the tail.)
How do you think they know if they are north, south east or west of their destination? By the home molecule smell?