Also, when the study tests one pool in one waterway, once, it doesn't paint a complete picture. These are organisms - they wax and wane, move, die off, grow again, propagate under certain conditions and disappear under others. The waterways are constantly fluctuating in flow, temperature, chemistry, etc. in tiny ways.... The system is changing all the time. To claim any waterway free and safe is ridiculous. It might be today. It might be tomorrow. No way to conclusively stake up a sign and say "drink! be healthy!"

I am posting to provide information - I don't care about theories and I'm sorry my information offends sandia, but one person's insistence that treating water is unnecessary just offends all sensibility. No person or entity can possibly expect to be correct on this point. The CDC, the park service, the forest service and the state parks will all tell you - it's not a guarantee that the water is clean. And they are correct. Sometimes they will tell you with certainty that it IS NOT clean, and DO NOT drink without treating. The water along the Ohlone Wilderness trail (cattle range! eeeew!) is all accompanied by signs next to each faucet to not drink it without treating it. Yosemite does not post a SAR person at the top of Nevada Falls on summer afternoons with a big ol' Katadyn filter and a five gallon bucket for no good reason - tourists who get there will dip their nalgenes and bottles into the Merced, which tested positive and is probably still positive, off and on, for e. coli. If you spend any time hiking upstream there, you'll see bunches of nekkid tourists in the water. Filter the water!

There is no way to predict what's upstream of you. No way to know who did what in the water minutes ago. No way to count the deer, llamas, goats, cows, horses and mules that marched through it with poop-covered hooves. You just don't know and until they make a very light and effective way to test it, you will NEVER know for sure what is in the water.

What do I know? That many of my fellow SAR folk treat it as an occupational hazard - you might get giardia. Shrug. Yeah, it's not life threatening, really. It's a darn nuisance. Me, I would much rather spend my sick time (same as vacation time for me) backpacking than suffering projectile evacuation from both ends and taking antibiotics that screw up my digestive tract further. (UGH.)

So, yes. I will tell you to treat the water in the central Sierra Nevada, absolutely. It would be potentially cruel to not say so - do I want my friends to go through this sort of thing? Not on your life. I like 'em. So I loan them my filter when they don't have one.