If you're wondering whether to bring purification tablets, a water filter, or just drinking out of the stream, this paper may help. Although the paper's specifically written about the Sierra, the information should be applicable to the local cascades as well. http://lomaprieta.sierraclub.org/pcs/articles/giardia.asp most interesting quote from the paper:

"Recall that San Francisco water can contain a concentration approaching 0.12 cysts per liter,26 a figure now seen to be higher than that measured anywhere in the Sierra. San Francisco city officials go to great lengths to assure their citizens that the water is safe to drink, and if true-as it most assuredly must be-this comparison alone is quite revealing."