Ah, that’s an interesting question. I assume you really aren’t trying to carry water, but instead are taking short stops at streams to fill up.
The Hiker Pro
Some filters are pretty light. The Katadyn Hiker Pro ($80), for instance, weighs just 11 ounces. Not intolerable if you carry a fanny pack, which surely you do in order to carry some snacks and a light jacket. But then, you specifically reference a purifier, which by definition kills viruses and things like that, in addition to the usual giardia and other water-borne bugs. The thing is, I am not entirely convinced you need that in the Cascades. At least, I’m not aware of any reports of water-borne viruses hitting hikers around there. So I think in most cases a filter alone is fine.
You could alternatively carry an MSR Miox Water purifier ($140), which uses an electrical charge to create a salt-based purifying agent. It kills viruses and bacteria in 15 minutes, giardia in 30 minutes, and cryptosporidium in…yes, four hours. Which is a problem, as crypto IS something you might encounter. And it’s something that a filter would whack pretty easily.
In my view, to be totally safe, you carry a light filter, then a bottle of MSR Sweetwater Purifier ($10, 2 ounces). The filter gets the things such as giardia and crypto, the purifier whacks the viruses—and does so very quickly. You can filter, add the purifier, give your bottle a few shakes, and drink.
Stay safe out there!