As you continue to climb, you continue to sweat, and part of you now wishes that the nice day would cloud up and cool off a little. The next time you reach for your water bottle, you see that it's almost empty. You wonder whether you should save it until you reach the next water source, but you know that you shouldn't ration water if you're thirsty because dehydration can impare future decision-maiking. So you finish it off and continue. After fifteen more minutes of tough hiking, you're relieved to come to a pleasant waterfall...

You drop your pack and stick your head right in the icy water. Though the water is so cold it's almost painful, it's also very refreshing. You splash some more on your face and turn to your backpack to fill up your bottle, immediately realizing that you forgot to pack your water filter within easy reach. It's somewhere deep inside your pack, and getting to it will mean unpacking everything. You turn back to the clear water and wonder if it would be safe to drink untreated. You remember reading something in an outdoor magazine that said there isn't as much of a risk if you drink cold water from fast-flowing streams at high elevations. Do you: