Having grown up in Scotland and now living in New Zealand I have always just taken my water straight from mountain streams with no ill effect. So I was wondering what's in the water there in the USA, especally in the mountains that warrants using a filtration system?
9 out of 10 times, mountain water in the U.S. is fine, particularly if you are not on a piece of land that allows for grazing.
However, you can occasionally pick up giardia, particularly if you are in lower terrain that has beavers or other water loving mammals.
I know a number of folks that generally don't treat their water in the mountains, and very few of them get sick. However, chlorine dioxide tablets are so light and effective that I just use them even if the risk is small.