There is a lot of cattle grazing in the mountains. Even at 10000 ft. Probably the higher you go up the safer the water, but where I hunt, the cattle graze up to the peaks of the mountains at 12,000 ft. Its better to be safe than sorry. A bad case of the sh----ts will change your mind about not purifing creek water in a hurry. Not to mention the stomach cramps that double you over for24 hours straight. Its not a hunting story you would ever want to tell again.