The further upstream and the closer to the source of the water, the better the chances of it being uncontaminated. However, don't treat all springs as pure sources - you just never know.

I personally like boiling. It is the most broadly effective method, although it does use up your fuel. If you can't trust a ranger, whom can you trust? Truthfully, they usually give accurate information, although they will tend to be cautious and conservative.

Finally, if you are desperate for water,and out of treatment materials, go ahead and drink. Whatever is in the water can be dealt with once you get back to civilization, but dehydration is very deadly! This from an old desert rat....