treating water in the alpine

My understanding is that giardia cysts are not buoyant, thus settle out in still water. Finally contracted giardiasis after fifty years, now using a steripen.

This^ Everyone seems to gravitate towards fast moving water, but my understanding has been that clear, slow moving or calm water is your best bet; all dirt and parasites settle out to the bottom.

I would have to go back and read more to be sure, but I thought leptospirosis was a bit more likely in still water. But it dies in cold water, so should not be an issue in cold, still water. But then, filtering is not a reliable approach to lepto-prevention as they can corkscrew through a fine filter.

Here is everything you want to know about giardia
www.cdc.gov/parasites/giardia/

BTW according to the CDC, you can get giardia from a properly chlorinated swimming pool, it takes 45min for the chlorine to kill off the cysts (they have a hard shell that protects them).

I read somewhere that drinking from a fast moving stream is worse because the running water mixes it all up. But if you drink from a lake and only tap the first few millimeters of water at the surface, the sun will have purified the water on top.

I read somewhere that drinking from a fast moving stream is worse because the running water mixes it all up.  But if you drink from a lake and only tap the first few millimeters of water at the surface, the sun will have purified the water on top.

That's my take, too. A lake would be safer. I think I got my information from a fellow that took my avalanche course years ago. I met him near Anderson Pass and we talked Giardia. He had been doing research on it. He also said that he tested boiling and Giardia broke down almost immediately, even at the boiling temperature of Mt. Mckinley (20000'). At 145 degrees it was gone. I use a filter.

I've had Giardia three times, twice in Washington, once in Utah. One time I think I got it from a stream in the "Hairpin" valley north of Kangaroo Pass.

... only tap the first few millimeters of water at the surface, the sun will have purified the water on top.

The water on the surface might be less infected due to settlement, but I wonder about the sun purification idea. UV light does indeed kill bacteria, however the operating instructions for the Steri-pen device indicate that the surface tension of water makes a very effective UV barrier -- without it the UV light emitted from the Steri-pen would "sunburn" any skin exposed.

OTH I've read that certain plastics -- PET for example the plastic used in 2-liter pop bottles don't block UV, so water in such a bottle, left in direct sun for several hours will be sterilized. en.wikipedia.org/wiki/Solar_water_disinfection