Here's a question for Salmo and the other bios. I remember discussing the possibility that, before hatcheries and development, that PS Chinook were primarily Summers and not Falls. Late summer natural flows in most PS streams get pretty low. And probably warm. Maybe a bad situation for a lower mainstem spawner.

As the Green River fish got spread around, and dams on mainstems at least gave cooler water (and now we regulate the flows for more fall water) we created the PS Fall. They have adapted, survived, and spread.\

A similar thing happened in the Sacramento when stream temps downstream of Shasta and Keswick lowered, making that reach more suitable for salmon spawning.