I'd have to believe in the summer, more water flows out of the north fork at Hoodsport then actually down the river itself. I may be wrong on that as I don't have figures to back it up.
I think Tacoma did increase the flows down the north fork a few years ago.

I mentioned the septics because I have access to several properties on the canal for fishing. In the winter and late spring, you'd be amazed at how high the water gets on the king tides. Septic systems in some areas are actually underwater.
Combine a big tide with a bunch of rain and a stiff s / sw wind and it is pretty ugly.
With the lack of flushing beyond the big bend, I wonder how much organic material builds up. It seems this is happening earlier each year, but I'm sure weather and lack of rain play a roll this year.
SF