They return to the stream they grew up in before they went to sea, not necessarily the one where the eggs were hatched. It's very common to stock streams with

They return to the stream they grew up in before they went to sea, not necessarily the one where the eggs were hatched. It's very common to stock streams with fry from a hatchery located somewhere else. Salmon generally spend a year or more (much more, in some species) in fresh water before migrating to the ocean. That's plenty of time for them to imprint on the "foster stream" -- they'll return to it, no problem.