The current, up to date and in the literature information is that, in the anadromous zone, steelhead and rainbow are the same damn fish. Some offspring of anadromous parents stay in freshwater, some offspring of resident parents smelt and go to sea. Further, the factor that leads to smolting includes temperature and flow. Colder summer temps and higher flows lead to residency.

Even more interesting is that some rainbow, resident upstream of waterfalls, have become anadromous when placed into anadromous waters.

Those fish are significantly more complex than NOAA is wiling to admit. If they were too admit that a resident rainbow in the Cedar (for example) and a steelhead in the Cedar were the same damn fish and the river was plugged with "residents" (as it was recently) then listing that fish as endangered or threatened becomes a rather long stretch. Lots of politics involved in the listing I'll wager.