Stream water looks clean until the week after

The job

Turn water from a mountain stream into water you can safely drink. Two guys, one creek, one afternoon.

How the rookie does it

2:10 p.m. The rookie reaches the creek, kneels where the trail crosses it, and dunks his bottle. Twenty feet upstream, a golden retriever is drinking and enjoying it. The rookie decides this is a good sign.

2:12 p.m. He holds the bottle to the light. Perfectly clear. Cold enough to hurt his teeth. He reasons that nothing nasty could survive in water this cold, and he's wrong about that.

2:15 p.m. His wife had tucked a bag of purification tablets into the pack and said, "Just in case." He'd said, "We won't need those." He finds them under the socks. He decides they're a monument to her worrying and leaves them there. She was right, and he'll find that out later.

2:20 p.m. He's brought a squeeze filter. He fills the dirty bag, screws on the filter, and squeezes into his clean bottle. The tip touches the creek water on his fingers. Then it touches the rim of the bottle. Then he sets the filter on a rock to take a picture.

9:40 p.m. It's 28 degrees. The filter is in the side pocket of the pack, outside the tent, where it can think about its choices.

Eight days later. He's at home, in the bathroom, and he has stopped being able to link it to the creek. That's the trick with this stuff. It takes its time.

How the veteran does it

Call him Walt. He's walked these trails for thirty years, and he spends about six minutes on the whole job.

He skips the trail crossing and walks fifty yards upstream, past the spot where the dog has been, to where the water runs fast over a rock. He fills the dirty bag mid-depth, not off the silty bottom. It's a little cloudy from last night's rain, so he sets the bag on a flat rock for ten minutes and pours it through a bandana into a second container, leaving the sludge behind.

Then he hangs the bag from a branch and lets gravity push it through the filter. He keeps the filter's clean end in his shirt pocket the whole time. Hands washed first. The clean bottle never touches anything but clean things. He drops a chlorine dioxide tablet in to cover viruses, sets a watch alarm, and eats a handful of peanuts.

At night the filter goes in his sleeping bag. He's done this since the first one he ever owned cracked.

What the veteran knows

1. Clear isn't clean. The bugs that matter are Giardia, Cryptosporidium, and bacteria like E. coli. You can't see any of them. Sickness shows up a week or two later, long after you've stopped suspecting the creek.

1. Pick the spot like it matters, because it does. Choose moving water over still. Go upstream of trail crossings, campsites, and anywhere animals gather. Skip water below farms, grazing land, or old mines, and skip anything with a sheen, foam, or a dead animal in it. A filter doesn't remove chemicals. If you suspect runoff, find another source.

1. Clear the mud first. Silt clogs filters and uses up tablets. Let water settle, then pour it through a bandana or coffee filter. Dip from mid-depth, not the bottom.

1. Know what each method kills. A hollow-fiber filter rated at 0.1 or 0.2 microns stops protozoa and bacteria. It doesn't reliably stop viruses. Chlorine dioxide tablets handle viruses too, and the label says it can take up to four hours to deal with Cryptosporidium. Iodine is weak against Cryptosporidium. Boiling beats everything: a full rolling boil for one minute, or three minutes above 6,500 feet. In remote North American backcountry, a filter is what most people carry. If you're near any chance of human sewage, add a chemical step or boil.

1. Keep a dirty side and a clean side. The clean outlet must never touch the dirty bag, the creek, or your fingers. Rinse the threads of your bottle with treated water. This is where most people undo their own filtering.

1. A frozen filter is a dead filter. Ice can crack the fibers where you can't see it, and the water still comes out looking fine. Sleep with it, or keep it in an inside pocket on cold days. If it's been frozen, replace it. Backflush on the schedule in the manual and not by guesswork.

1. Always carry a backup. Tablets weigh almost nothing. Cold water slows them down, so give them extra time and follow the label to the minute. Filters clog and crack and get dropped. A backup is cheap.

One more thing, since it costs nothing. Camp and wash dishes at least 200 feet (about 70 big steps) from the water, and carry out what you carried in. Leave the creek better than you found it for whoever fills a bottle after you.

If you do get sick, a doctor can tell you what to do. Don't guess at it. If you're cooking dinner at camp and have to improvise, the same care applies to the pot, so it's worth knowing six knots to hang a gravity bag from a branch. And if someone in your group goes down on the trail, what to do in the ten minutes before the ambulance arrives is worth reading before the trip, not during it.

Your first time, done like a veteran

• Pack a filter and a backup (tablets or a small pot).
• Walk upstream of the crossing and any camp or animal traffic.
• Fill from moving water, mid-depth.
• Let cloudy water settle or strain it through a bandana.
• Wash your hands. Keep the clean outlet clean.
• Filter, then treat if you need to cover viruses, and honor the label's wait time.
• Rinse bottle threads with treated water.
• Keep the filter from freezing overnight.
• Camp 200 feet from the stream and pack out your trash.
• Tell your wife she was right about the tablets. She was.