Good question. I don't doubt that the range of this mysterious fish was once quite a bit greater. We know that before dams on the upper Columbia that they ranged into up the interior of Canada. Your question was up and back though and I have heard, but can't say for sure it is true, that the repeat spawning percentages for these far traveling fish is (and always was) very, very low.
Without looking it up I would have to say that steelhead, or any rainbows for that matter lived or migrated up to Missoula. They would have to go by way of the Pend Orielle River which had a couple of large falls (now dams) on it. I will have to consult Behnke on this.
If these falls were passible that would also open the way for the Priest River watershed. This is interesting and I will definately look it up.
Not steelhead, but here in MT they have had a tagged bull trout go downstream over Kootenai falls, up a creek to spawn, then come back up OVER the falls, which is quite a piece of water! If a bull trout can go over Kootenai falls, I would bet a steelhead can do a lot better.
The upper limits of migration for any anadromous fish on the Snake River was Shoshone Falls near Twin Falls, Idaho. Now, of course, Hells Canyon dam is the end of the road for them.
The longest migration in the mainstem Columbia was to the Kootenai River in Canada, possibly to the point where it loops back into the northwestern corner of Montana near Libby. Since rainbow are native to that part of the Kootenai, it's likely that steelhead at one time reached that far.
When Grand Coulee dam was built it permanently closed off over a thousand miles of spawning water in the upper Columbia and its tributaries to all anadromous fish. A friend of mine, whose father worked in the beach seine fishery on the lower Columbia in the the first decades of the last century (where horses were used to drag the nets in) says that the far-famed "June hog" chinook, some of which exceeded a hundred pounds, were all Kootenai River fish.
There is no evidence that Snake River and upper Columbia steelhead ever survive to spawn a second time. After all, the percentage that survive in the short-run coastal and Puget Sound rivers is usually quite small, rarely, if ever, exceeding 15%.
in response to preston's intellectual response would it be something to fight a 100-lb fish in the kootenai river. thank you preston as that was exactly what i was looking for. i knew someone would have the good stuff.
According to Behnke:
"Inland redband trout occur east of the cascade range in the Columbia Basin upstream to the Kootenay River in western Montana. They also occur to Albeni Falls on the Pend Orielle-Clark Fork drainage in western Idaho, to the Spokane Falls on the Spokane River in eastern Washington, and to the Shoshone Falls on the Snake river in southern Idaho."