Spawnout where steelhead spawn depends greatly on the riversystem and the fish themselves. I think that the main reason a lot of steelhead are mainstream spawners is because steelhead have been largely eliminated from small tributaries. here on the Washougal river most of the spawning occurs in tributaries, very small tributaries! The same tribs that the coho use. native steelhead in the tributaries below the hatchery hve been pretty much eliminated and wild steelhead above the hatchery are holding their own(barely)
This is the same with most of the river systems I have had the opportunity to look at. Steelhead will spawn anywhere they can find good habitat most of that habitat now occurs in the mainstream.