Pretty much what Poke 'Em said.

My experience in SE, Central, and Northern Idaho and here in Western Montana is that bull trout, cutthroat, and brookies tend to be "headwaters" species, that is, they need the really high quality water, cold, fast, and clear to do well.

Rainbows do just fine in the headwaters, but they do just as well downstream where the conditions may be somewhat degraded, and where the bulls, cutts, and brookies don't do well, or at all.

Further on downstream, with the least favorable conditions, browns will do better than the rainbows, if the rainbows even do well. The "headwaters" species hardly have a chance in those places.

If the native brookies in the east couldn't survive a particular system, it's not at all likely the cutts would.

John