In discussions with some OR bios they said that, at least in some of the more southern streams, the base geology is very different and provides significantly more nutrients into the water. This leads to more productivity.

WA base geology is nutrient poor. Rather than erosion providing the nutrients at some level, we need spawning salmon. Oops, can't do that.

This base geology thing shows up in trout fisheries where, like in the Idaho batholith, fish need to be 5-6 years old to mature while the same species may take 3-4 in a productive (limestone) stream.