Many people feel that steelheads and rainbows are exactly the same fish. In truth, they are similar but not the same, despite the fact that they both belong to the same species. There are several differences between the two, both in appearance and in habits.
Steelheads are much more like salmon than they are like rainbows. This is because of one key; they live a portion of their lives in the ocean. Rainbows do not. Yet, steelheads aren't quite like salmon, either.
With salmon, the young fish move almost immediately downstream to the ocean, where they grow and mature, returning to the rivers and streams of their birth in order to spawn (lay eggs). Young steelheads, on the other hand, remain in fresh water for around a year, sometimes longer. Only then do they work downstream to the ocean. This gives them quite an advantage because they are usually larger and stronger before they reach the ocean, so are slightly less likely to be eaten by predators.
Rainbows remain in the same bodies of water through their entire life cycle, never going to the ocean. The body of water can include lakes, naturally, however they can also include streams and rivers. This is an important distinction because a rainbow may have access to the ocean, but will not leave the river or stream. There are instances, such as in lakes, when rainbows cannot move freely to the ocean. However, in many great fishing rivers, it may be possible for an angler to catch a nice rainbow in one cast, and a steelhead with the next.
It is similar to the difference between "country people" and "city people". They are different in several ways, most people would agree, yet there is rarely anything preventing a country person from moving to the city or visa versa.
The two lifestyles usually change the appearance of the two fish, though at times it still takes a fish biologist to tell them apart. A steelhead most often becomes much more silvery in color, sometimes almost hiding the spots and colors rainbows are known for.
Steelheads nearly always grow larger as well. Steelheads can weigh in excess of 50 pounds, while rainbows seldom become heavier than 10 pounds, and most are much smaller than this. The reason for the difference is that in the ocean, the steelheads have access to a greater amount of food than the rainbows typically have in lakes, rivers, and streams. The change can happen rapidly, too, within only a couple years.
Fishing for the two in rivers and streams is similar, except for the difference in weight and the fact that steelhead generally have a spring and fall spawning run. Rainbows can be caught nearly any time of the year. Nearly all steelheads are caught only during the runs.
Because of all of this, and physical difference, many scientists classify rainbows and steelhead as two distinct subspecies. Some don't and say they are the same species. Regardless, for the fisherman, tying into a large rainbow is an entirely different experience than catching a large steelhead.
Catching one of each is usually enough to convince a person that these are not the same kind of fish, even though the species may be the same. To the fisherman, the difference is about the same as catching a big large mouthed bass and catching a large rock bass. Both are thrilling to catch and to land, but they just aren't the same thing.