If a steelhead is a rainbow trout, then why not some steelhead stay in fresh to become a rainbow an vice versa? I believe they are different fish with very close genetics. As I remembered working at Puyallup's Trout Hatchery when I was a kid, I had witnessed vast difference in behavior b/w rainbows and steelhead. When you walk near their ponds, the rainbow fingerlings are always friendly and curious; they would come toward you and follow you around. As for the steelhead fingerlings, if they see you, they would head the opposite direction. At feeding time, the rainbows would go on a feeding frenzy like pirahnas. The steelhead would mostly feed on sinking food and were not as voracious feeders. So how can they be the same fish if they behave so diffently? If you happen to visit a hatchery, check this out for yourself then contemplate your findings with the literatures which claim that the two fish are the same. One last note, if they are the same, then they should mingle freely together at spawning time, right?