That is a coho jack. Coho typically smolt at about 4 inches in length in spring and spend 1 winter in the ocean before returning the next fall---a total of ~1.5 years. That fish smolted this spring and thus only had ~5-6 months to grow from 4" to that size. I say it is a coho because a) it has a deeply forked tail, which steelhead do not, and b) the spotting is sparse on the dorsal and almost absent on the tail (like a coho, not like a steelhead), c) that is way too big for a steelhead smolt anyway (some resident rainbows that size though). A steelhead smolt will look just like a small rainbow trout at this time of year (~6 inches in length), and will only get silvery in the spring just before leaving (~6-8 inches). If there are alot of coho jacks around that bodes well for next years run of regular size adults!