Some of those lower tributaries were likely overwinter rather than just spawning. Steelhead and coho move around a lot, especially in the fall. I saw a paper that looks at, I think, the Snohomish system and concluded that the diking, leveeing, filling, removal of beaver ponds and such in the lowlands lowered the basin coho smolt protential by 2/3. Probably knocked down steelhead too. And, might as Rivrguy what the Chahalis watershed would look like coho-wise if the lowlands were a complex of beaver ponds, sloughs, and such.

Oddly enough, in the late 70s my wife and I looked into buying a private hatchery in the Much Creek watershed. Might have been the one you know.