I believe that the reason for the double peak in chinook harvest is also due to the high effort of commercial fishing.

Through the late 1800's almost all the chinook commercially harvested in the Columbia were highly valued Spring Chinook. Around 1910 or so, the population of springers was in serious, serious decline due to overharvest. Commercial fishers then swapped to Fall Chinook, much less valuable commercially, but still available in fishable numbers.

Initially after the switch from fishing over a depressed run to fishing over an underutilized run, the harvest rates shot up...and then started to decline due to, big surprise, overharvest.

Then the dams came along and really put the screws to both the runs of chinook.

All this info can be found in "Salmon Without Rivers" by Jim Lichatowich, and is backed up by reference to documents outlining the timing and amount of harvest throughout those years.

The above writing by me may not be totally accurate, as I don't have the book here with me, but to the best of my memory, that's what happened. I check the book later, and if something different presents itself, I'll correct my above assertions.

I think it's pretty clear that dramatic declines of Columbia River chinook were taking place before the dams were built, but that the dams really took the ball and ran after their construction.

Fish on...

Todd.