Another alternative is to use Katadyn's Micropur.

Purifying water couldn't be easier, simply drop a Micropur tablet into a quart of water; drink with confidence and no chemical aftertaste.
[ul][*]Features the same proven technology used in municipal water supplies; effective against viruses, bacteria, Giardia and Cryptosporidium[*]Destroys viruses and bacteria in 15 minutes, Giardia in 30-minutes and Cryptosporidia in 4-hours[*]Meets the US EPA purification guidelines; active ingredient is chlorine dioxide[*]Each tablet is individually wrapped and sealed[*]While a 4-hour wait time may be inconvenient, alternative disinfectants do not claim to destroy Cryptosporidium [/ul]
To protect consumers who may not be able to determine if the water is cold and turbid (dirty), the EPA requires product instructions to have a 4 hour wait time.

While a mountain stream is cold...it probably isn't dirty. Katadyn says a 30 minute wait is all that is required even for Crypto.

Bilbow hit it right on the head...Bucky the Beaver might be legs up not 50 yards upstream from you.

You can pick up a 30-pack at REI for around $15.

Regards,
Steve