Even drinking from clear mountain springs can potentially be harmful to your health, if you don’t filter or purify it first.
Probably not... In fact, likely not. The only real waterborne disease to look out for in the mountains is beaver fever - Giardia - Which is not found at mountain springs, or even in white water (cold and over-oxygenation kills the bugs)... Pretty much, don't drink from water where beavers are. Never behind a beaver dam, or directly down stream. Slow moving water and lakes are suspect.
If you are from the area and used to drinking natural water, you'll be fine, but if you wind up here by chance, or never drink anything but city water, the danger is more pronounced. It will make you sicker than a dog for some days, but that is not the water's fault. That is your fault for having a gut unable to handle it, because you have been drinking dead city water all your life.
At the very least, you should always boil your water first before drinking it.
The very least? As far as bugs go, boiling is the very best. Make SURE your kit has cooking and carrying gear made of tin or stainless, capable of putting it right in the fire. No plastic canteens, no plastic water bottles or bladders. If your gear is fire-worthy, boiling is your best bet, long term. Water-straws and filters are alright, but they can plug rather quickly, and purification tablets run out even quicker... but a tin canteen and a tin bush pot will go a long, long time.
Toxins are another thing - always look for where the critters are drinking. If you see no sign of critter activity, or if the place has dead critters nearby, of course don't use the water there. That water will be toxic. BUT, just because the critters are drinking, doesn't mean the water is safe for you (critters drink from behind beaver dams all the time).
Best to drink from a fountainhead (spring), or a seep where the water is coming out of rock, or from glacial runoff... It has already been filtered. But in a pinch, if suspicious, find a sand shore or bank, walk three long strides from the river, and dig down to the water level. let your pit clear, and that water will be good.
