Hi Foy, I think it may be due to the particular color of that watersheds serpentine rock. It seems more blue than the typical green. You can see it in the fragments at the surface and I think it translates to the water color. Not sure of the particular mineral though.
Could be from weathered serpentinite rock. The Franciscan Complex outcrops in the Oregon Klamaths and is a "melange", which can be described as scrapings from the slaughterhouse floor in that its rocks were scraped off of subducting oceanic floor and mashed up against and accreted to the western edge of the North American plate. There is a lot of serpentinite bedrock in the Fransciscan. Never heard about weathering serpentinite rock coloring streams, but that means nothing--I'm dead last in knowledge of the Franciscan and most West Coast terranes.
The rock flour we see in other environments is created by and delivered into the streams by actively moving glaciers, so flour-stained mountain streams often remain stained year-round or at least from early Spring to late Fall as melting from the glacier provides a steady supply of rock flour. But if it's weathering serpentinite providing the staining, obviously no glacier is required!