There's not a lot I can add after reading C'man's and Smalma's posts. Generally, yes, the Chehalis (and OP) steelhead runs are following down the same path as the severely depressed Puyallup and Nisqually populations. I think it isn't for exactly the same reasons however. The OP rivers have the most intact freshwater habitat in the state, so it's fairly logical that those populations are the last to collapse. I think the Chehalis basin is kind of in between the OP and Puget Sound (PS) rivers in terms of freshwater habitat quality. The biggest difference affecting steelhead runs that I can see is that PS river populations have further to travel in estuarine-like waters before reaching the open ocean.

Smolt tagging studies over the last decade illustrate that heavy predation near the river mouths and in PS by marine mammals and some birds is severely limiting the proportion of the steelhead smolt population that makes it to the open ocean. Some of that same kind of predation affects OP and Chehalis steelhead too, but not nearly to as great an extent. We can see in PS populations that the further the distance from the river mouth to the open ocean, the higher the smolt mortality, with the Nisqually River, as the southern-most PS tributary, being the most severely affected.

The recent devastation to OP and Chehalis basin steelhead runs appears related to declining productivity in near-shore ocean waters - like the poor upwelling event that was documented just a few years ago - and more open ocean declines like the large "warm patch" in the central north Pacific (which happens to be the area most of our steelhead populations migrate to), and the potential impact of over-grazing of the north Pacific by massive numbers of Alaskan and Japanese hatchery chum and pink salmon releases, which also migrate to and forage in that central north Pacific zone. The apparent declining productivity there sure seems like it could be a final chapter in a doomsday scenario for PNW steelhead.