Quote Originally Posted by Preston Singletary View Post
The origin of the term "steelhead" is still something of a mystery. Some of the earliest sources indicate that the name was applied by commercial fishermen (back when there was still a large commercial harvest) because the steelhead's generally heavier and stronger bone structure (including its skull) required two or three whacks with the fish club to apply the coup de grace; other members of the genus Oncorhynchus required only one. Of course the steely, blue-gray color of a fresh-run steelhead's back and head may have contributed to the popularity of the term.

The twenty-inch rule (in Washington) is an (admittedly lame) effort to differentiate between large resident rainbows and steelhead and, as such, has no particularly rational basis. A twenty-inch resident rainbow in most of Washington's anadromous rivers, while potentially possible, is very unlikely. It does provide endless fodder for fishing forums in the form of: "Was the twenty-inch rainbow I caught on the Yakima River (where such a possibility does exist) a steelhead or a resident rainbow?"
I do not know if the 20 inch rule (or 16 inch rule) is lame, I think the dfg will readily admit it is just a practicailty. The only way to tell if a 16 inch rainbow (in anadronymous waters) is by a scale sample, which a angler is unable to do.