Originally Posted by forrest_m

My rule of thumb is that I treat everything unless I'm above timberline and I can see the water flow coming out from the snow it is melting out of. I've broken this rule a few times when I was really thirsty and nothing bad has come out of it, but it's really impossible to tell visually whether the water is safe to drink. The main risk is giardia, a GI parasite that is very unpleasant. Giardia can be spread by wildlife AND can remain dormant even in icy cold water for years; tests have found giardia in very remote lakes and streams in the cascades and sierras. Since an iodine tablet sufficient to purify and a bike bottle is about size of a lentil, there is little reason not to carry a couple. Your best bet for filling bottles is not the big waterfalls, but smaller creeks and streams (the kind that go under the road in a culvert, not a bridge), since they are almost always easier to get to. Try to go to the uphill side of the road so you aren't drinking motor oil.

In the lowlands, all bets are off. There are all kinds of chemicals in agricultural runoff, so I wouldn't drink stream water in farmland even with iodine unless I was truly desperate.