A couple of extra comments.
Just for the mountain streams.

I would usually drink the water from fresh streams at high elevations - coming from the snow melt. Just natural water ... no filtration. I never had any problem with that.

I was always suspicious of the water once it descended to the altitude where there were cows grazing. Sometimes they are up quite high - in the high pastures. I didn't usually trust the water at that stage ... without purification. I thought it was probably a personal quirk on my part - I had no evidence that such water was bad. So I found the recent comments on this thread that giardia is coming from the cow patties to be quite interesting.

Finally, I had an offline conversation with Ron Hood one time. It's too bad that he is gone. He was a really good person for survival techniques. I should have taken one (or more) of his courses. Anyway, he told me that it it possible to adjust your stomach to become "resistant" to the giardia. Over time you can condition your body so it does not cause sickness. I would say that it's true - because he didn't give such advice without trying it himself. But also I think that medically it's not good to have a lot of exposure to the giardia - it can cause long-term problems (somewhere I remember reading that).

SO: you can drink from high mountain streams where the water is fresh snow melt. And if you have been thinking about taking a survival course from somebody really good - put it on your Bucket List and GO and do it! These people are not around forever :-)

Pete2