Here is part of what lead author (Derlet) of the study linked above had to say, as of about a year after the samples were made:

"Should you treat your water to kill bacteria? This is a personal decision, and depends on the risk the individual wishes to take. I believe the risk is low and treat my water in only two situations: 1) below cow and sheep pastures; and 2) in slow flowing, warm streams immediately below places humans may be swimming or bathing"

Derlet goes on to say....."If one wants to be entirely safe, one could purify water but my suspicion is that perhaps less than 1% of streams in the Sierra would have Giardia significant enough to cause infection in humans."


It may be possible to simply choose sources that don't involve the situations to which the author Derlet refers. Also note that his comments and research refer to backcountry water in the Sierra Nevada


Here is link to article I'm quoting from

http://www.yosemite.org/naturenotes/DerletWater.htm