CM-
The information that I have stumbled across over the years indicate that say a 100 years ago the early time Chinook (spring/summers) dominated the runs. That said on most Puget Sound streams the not sure the divisions between spring/summer/fall Chinook stocks are as clear cut as one would like; at least in terms of river enter timing.

While it is probable that hatchery programs have influenced the return timing across the region I don't think that was the only selective factor in play. A century ago the early timed Chinook were not only more abundant they tended to be larger as well. Given the early availability and high table quality Just like on the Columbia the early commercial fisheries likely target those high value fish and rapidly fished the populations. Just one example records from the early 1900s indicate that early season (May) gill net seasons (non-treaty) took palace on the North Fork Skagit with records of catches in excess of 15,000 just for that month. An indicator of the potential size of those early Skagit fish was the gear of choice was gill nets with a 9 1/2 inch mess. The fishery selectivity continue later in the century moved further into the salt with higher exploitation on both older and early returning fish.

Another factor that have (and continuing to) that has dictated the life strategy that works best for various PS Chinook is habitat changes. Traditionally early Chinook tended to spawn earlier and further upstream in Puget Sound basins. Dam construction has always higher affects on upper basin populations that those spawning further downstream. I have come to believe a major factor determining spawn timing (and indirectly run timing) is stream temperature profiles. While in larger systems traditional spring Chinook were those that had either long migration journeys or had season migration barriers that selected for that early migration. On PS rivers it is rare for Chinook to penetrate PS basins more than a 100 miles or so and SG indicated those Chinook barriers were rare. However that Chinook that did penetrate the upper basins for spawning were typically in time race to have their eggs development to race a high survival stage before stream temperatures drop to lethal levels for the developing eggs. Chinook eggs have to develop at least to the "eye stage" before stream temperatures drop below 4 degree C. At least in the basins of North PS (the ones I'm most familiar with) than means spawning needed to be completed prior to say early September and substantially earlier than those Chinook spawning further down basin.

With climate change that spawn timing of the early stocks appears to be changed. The spring Chinook of the upper Sauk (a wild stock with relatively low hatchery influence) 35 years ago typically spawned from late July to early/mid September. Today the spawn timing has moved later; from mid-August through October. That timing is more typical to once was considered the "norm" for summer populations.

I do remember reading that one or twice fall Kalama Chinook were transferred to more northern basins in the 1950s or laters.

Curt