I recalled a local (SJMN?) news story a few years back about a perfessor who was measuring pathogens in Sierra water, going back yearly at numerous locations.
I didn't find the story, but did find the guy who I think was mentioned in the article: Dr. Robert Derlet, PhD.
There's some interesting stuff on rei.com, of all places (the number one profiteer on water purification systems, I'll wager): http://www.rei.com/learn/expert-advice/water-researcher-QA.html.
Also, another article he authored (not a research paper, by any means): http://www.pcta.org/help/join/magazines/SierraWater.asp
That article mentions that they couldn't find giardia cysts in the manure of Sierra pack animals, which I found surprising. In the REI page, Derlet says he would treat water downstream from cattle grazing. But perhaps that's due to concerns over coliform or other bacteria.
I haven't actually found any quantitative reports from his giardia-counting efforts, save for the "you'd have to drink 250 liters to get sick" statement, which maybe implies one cyst per 25 liters, or .04 cysts/liter, assuming he's using the "10 cysts minimum myth. I'll concede that the 10-cyst thing is a bit of a myth. Note that that does not invalidate the hard number contained therein: .04 cysts per liter.
There seems to have been much discussion here, in the past week, of the question of how to interpret the words of the one Colorado study. It has been claimed that Rockwell's paper has been "debunked." Has anybody tried to debunk that Colorado study?
It seems easy to pick nits with details in any study or article. I will go back and try to re-read the debunkification of the Rockwell paper. But has his basic thesis been debunked? That might be summed up as:
1. Giardia is everywhere. Has that been debunked?
2. There isn't (much) giardia in Sierra streams. That seems bunked, still, so far as I can tell. See Derlet, above.
3. You aren't likely to get giardia from drinking the water. Has that been debunked? How do you reconcile that with the absence of giardia in the water (assuming that's not debunked). See also articles to the contrary (e.g., "Cyst acquisition rate for Giardia lamblia in backcountry travelers to Desolation Wilderness, Lake Tahoe").
4. If you get giardia, you likely won't even know it. I've cited one study already that supports that claim.
5. If you get sick, it is most likely something else. See the paper cited under 3.
While one can pick holes in specific statements in the midst of this argument, that doesn't mean that the argument doesn't still hold up. The flip side of "even a stopped clock is right twice a day" is that "even an argument with a couple of flaws can still lead to a valid conclusion."
Perhaps the OP's biggest mistake was the admonition "Let's talk SCIENCE." It's not really a scientific question. It's a question about the level of risk one is willing to accept.
BTW, somebody above said: Still unanswered, my original question that started my readings: how long does a giardia cyst live in the air, out of water or outside of fecal matter?
That very question is answered plainly enough in the very report that has been roundly criticized (and allegedly "debunked") here. Most loudly, ironically, by the guy who asked the question. I wonder if he even read that report.
Here are the citations for that particular detail (I won't repeat the "debunked" answer to the question; look it up yourself).
DeReigner D. P., et al: Viability of Giardia Cysts Suspended in Lake, River, and Tap Water. Applied Environmental Microbiology, Vol. 55 no. 5, 1989
Olson, M. E: Human and Animal Pathogens in Manure. Conference on Livestock Options for the Future. Winnepeg, Manitoba, June 2001