Boil it.

Boil it. Rolling boil for one minute (three if you’re above 6,500 feet). That kills bacteria, viruses, and parasites. No filter, no tablets needed.

Boiling won’t remove sediment or chemical nasties, but for a clear-looking mountain stream that’s your biggest worry. If it’s silty, let it sit in a bottle for an hour, pour off the clear water, then boil. Or strain through a bandana or coffee filter first.

If you can’t boil (no fire, no stove), sunlight works but takes hours and needs a clear bottle and full sun. That’s UV treatment — leave it in direct sun for at least six hours, or two days if cloudy. It’s reliable, just slow. And the water must be fairly clear for UV to penetrate.

One more option: household bleach (unscented, no additives). Two drops per quart, shake, wait 30 minutes. But you said no tablets; bleach is basically the same thing chemically. I’d still pick boiling.

Boiling is your Plan A. You’ll never regret the five minutes it takes.