From what I hear the Sierra's water is good to drink from the stream. Is this always the case? Or is it only good past certain elevation?

No clear answer

Many threads about this including the recent http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=68224

and Rog's article from a few months ago

Two things to consider:

1. A healthy, non-immunocompromised adult can tolerate a certain degree contamination by the usual nasties (cryptosporidia, giardia, et alia). This is exactly why we evolved immune systems.

2. A bottle of water purification tablets weighs a fraction of nothing.

For my part, I've drank a lot of water directly from the source, and I've never had a problem. But I seem to have a pretty robust immune system, and for what it's worth, I only drink untreated water if it "seems clean" (which, I admit, is a terribly unscientific criterion for a scientist to use! heh). But I am also aware that each time I take a drink from a rushing mountain stream, I am playing a game of gastrointestinal Russian Roulette.

Edited by El_Jefe on 09/19/2012 13:10:24 MDT.

Last High Sierra trip I took:
5 days, 5 people, nobody treated a single liter of water that I am aware of.

Everybody was fine.

Skill?
Luck?
Divine intervention?
Science?
Stupidity?
Arrogance?

You decide.

To advocate for devil, Craig

If there is a small chance of getting sick from 1 day, say 1%

Then for one trip you'de have to be unlucky to get sick

But if you got sick 1 of every 20 trips you'de want to treat your water


To add to your list:

Statistics?

The higher up you are, the closer you are to the source and there is a less chance of having a dead animal upstream.

I just got over giardia that I'm pretty sure came from a recent sierra trip (SEKI). I usually don't filter and just try to get fast-moving, high-volume water up high. My luck ran out this time. The nausea and weakness I experienced were unpleasant enough that I am resolved to filter and treat my sierra water from now on. Not only is giardia unpleasant, but you don't want to have to put your body through a round of antibiotics if you can avoid it.