That's according to an ER doc and professor at UC Davis named Robert Derlet, who has been testing water at the most popular Sierra wilderness sites for years for the Wilderness Medicine Society.
Here's a study on the subject he authored a few years ago for the WMS, and here's a terrific story about his work from The Los Angeles Times, back in the good old days when they had an outdoors section.
Three take-away points: chances are good that you can drink water straight from Sierra streams and lakes without any filtration and stay healthy. The parasite giardia is not likely to harm you:
a shark, according to University of Cincinnati researchers who studied
the danger giardia poses to backpackers, namely "an extraordinarily
rare event to which the public and the press have seemingly devoted
(Derlet agrees, and in the aforementioned study points out that tests in the last few years on backpackers who did develop diarrhea in the backcountry found that giardia was not the cause. He's also skeptical that water filters, which easily clog, would successful remove giardia cysts.)
Second, the most likely risk is the familiar bug e. coli, which can be spread, for instance, by cattle manure falling in water, but even in heavily traveled backcountry sites the numbers are reassuring. Backpackers can safely drink water straight from backcountry lakes. Derlet told the Times reporter:
water is better from a nice, running stream because it's so fresh and
churned up. But the top few inches of lake water are zapped with
ultraviolet rays from the sun, which are a very powerful disinfectant.
Third, the most likely pathway to infection is by poor hygiene, and according to experts, washing your hands with alcohol hand gels after a visit to the wilderness privy is "incredibly effective" at preventing you from infecting others.
To yours truly, this fear of wild water is part of a larger fear of nature itself, and an absolute plague on American society… to help us overcome it, let me repost some beauty at Pioneer Basin from the astounding (and generous) wilderness photographer Buck Forester, a hero I have yet to meet…