Illustration by Steven Schalla, FlyFishingtheSierra.com, used with permission
Steelhead are sea-run, rainbow or redband trout which return to spawn after 2-3 years at sea (and typically 1 year as a smolt living in freshwater). They are genetically identical to resident rainbows and redbands, although because of different spawning patterns, steelhead and resident populations in any given stream rarely hybridize and may retain distinct characteristics. Steelhead that migrate to the ocean develop a slimmer profile, become more silvery in color, and typically grow much larger than the trout that remain in fresh water. Like Salmon, they return to their birth waters to spawn, and a few steelhead migrate from the ocean to spawning beds as far away as Montana and Nevada (via the Columbia River basin). Unlike Salmon, Steelhead can survive spawning, recover, and begin the migration cycle again.
Note that due to the ambiguity in identification, many states consider any rainbow trout caught above a certain size as “steelhead” (typically 16”). Wild steelhead are generally protected and must be released. Many steelhead streams are subject to special regulations. Check the state or tribal regulations before fishing.