Six questions I ask before drinking unpurified backcountry water

There was little doubt if I needed to purify the water from this stock tank on Arizona’s Coconino Plateau.

Earlier this month there was a healthy online debate over the true risk of drinking water from backcountry sources without purifying it first. I appreciate that Ethan Linck brought up the topic and agree that many sources are safe to drink without treatment.

But I agree more with Wes Siler and Christie Wilcox:

  1. It’s difficult to be certain of the contamination risk, and
  2. Waterborne cooties can be uncomfortable (understatement).
  3. So, as a general rule of thumb, just purify it.

Missing from their debate, however, was any instructional knowledge, specifically:

I’m generally competent in explaining the how’s of backpacking, so in this two-part series I will try to fill this gap.

If you’ve read The Ultimate Hiker’s Gear Guide, some of this text may sound familiar — it’s from the chapter on Water, which also includes discussion of contaminants and water storage. (A clause in my contract allows me to share short sections of the book.) If you haven’t read the Gear Guide, you should consider it: it’s filled with trail-tested advice, and almost certain to save you time and money.

They say to practice what you preach, but I’ll be honest: While I recommend that you purify backcountry water sources, I generally do not.

But your mileage may vary. Normally I hike off-trail and in low-use corners of the Colorado Rockies and High Sierra — at the headwaters of the Colorado and San Joaquin Rivers — where the water quality is top-shelf. I minimize the risk even further by being very selective about the exact sources from which I drink. In the less regular occasion that I’m backpacking in water-challenged environments like southern Utah or in high-use areas like the Appalachians, the rules of the game change.

In this post I will explain how I assess the quality and risk of a backcountry water source. But read it with a grain of salt: I have no academic training in this subject matter, and my approach is based entirely on personal experience, common sense, and a layman’s understanding of contaminants. So far, it’s worked out pretty well for me.

Before drinking unpurified backcountry water, I consider multiple factors. My ultimate decision is not based on a scoring system (e.g. “Three strikes and you’re out.”), but rather a holistic assessment of its quality and risk. If I deem the risk acceptably low, I’ll go for it; if the risk is too high, I’ll bypass the source or purify it first.

1. What is the distance to the source?

Water quality will typically be best at its source, like where it gurgles out of the ground or drips from a snowfield. Here, it is least likely to carry harmful pathogens like giardia or E. coli, or pollutants like heavy metals or pesticides. When it’s reasonable, I will hike uphill to the highest possible collection point — the top pool or pour-over, or at least beyond the “convenient” spot that everyone else is using.

Notice the streams flowing from the base of these talus fields in Colorado’s Indian Peaks Wilderness. That water is low-risk, because it’s so close to the source.

2. How much poop is upstream?

Most pathogens are transmitted by the fecal-to-oral route, i.e. you ingest water that has been contaminated by the feces of an infected carrier. So if I suspect herds of elk, families of beavers, grazing livestock, or hordes of other hikers upstream of my water source, that’s a reason for concern.

Look closely: mountain maggots (i.e. sheep) on Snow Mesa, along the Colorado Trail

3. What’s the volume?

In this context, the solution to pollution really is dilution — a water source with a low concentration of contaminants poses low risk. For example, in 2000, San Francisco’s primary water source — the Hetch Hetchy Reservoir, which is fed by the Tuolumne River in northern Yosemite — regularly tests positive for giardia. But the levels are so low that, on average, a person would have to drink eight liters of water just to ingest one cyst, a small fraction of the the minimum needed for infection.

The Napeequa River, in Washington’s Glacier Peak Wilderness. Miles from its source, it may have been carrying contaminants, but they would have been diluted by the spring snowmelt.

4. Is it flowing or stagnant?

You would think that a flowing water source would be better than a stagnant one: the flowing source is constantly flushed, whereas a stagnant source could be a one-way collection site for contaminants.

However, flowing sources are not necessarily better: they can carry upstream contaminants directly into my water bottle. I do not shy away from stagnant sources like lakes and potholes, but with one caveat: I try to take water from the top few inches, which get blasted with UV light, a proven purification method.

A large pothole in southern Utah

5. Is it crystal clear?

Turbid water is a bad sign. Microbes tend to burrow into sediment and other floaties, and the effectiveness of some purification methods (notably, chemicals and UV light) are compromised.

Silt-filled water that I let sit overnight. Chemical and UV treatments are less effective in turbid water.

6. What does it look, smell, and taste like?

I use my senses. If I see nasty-looking algae, I purify it. If I smell cow shit, I purify it. If I taste heavy metals, I filter it, or bypass it if I’m not carrying a filter that extracts heavy metals. This is just common sense.

A spring in Big Bend National Park. We were at the source, but the algae did not give me great confidence in its quality.

Questions drinking unpurified water, or am I missing an important factor? Leave a comment.

Posted in on February 27, 2018


  1. SusanS on February 27, 2018 at 8:29 am

    I hike in Canyon country a lot. One trick I’ve learned to help purify silty/dirty water is to carry alum, which you can buy in the spice aisle at the supermarket. It has coagulant properties so that it bonds with the silt & minerals and makes them settle to the bottom. You can then pour off the clean water for treatment. River guide use this trick a lot. Some have exact ratios, but for a 1-2 liter container, it doesn’t take much.

  2. sean on February 27, 2018 at 8:40 am

    Thanks for laying out some reasons, instead of just dogma. Another factor I consider is elevation — there’s more UV higher up. While I will drink from a Sierra lake at 10,000 feet, I’m less likely to drink from a Cascades lake down at 2-3000.

    • Andrew Skurka on February 27, 2018 at 8:42 am

      Hadn’t thought of that one, but it’s consistent with what I’ve seen now that you mention it. Another factor at play with lake elevation seems to be the nearby vegetation — typically you get a lot more life at lower elevations than high elevations, and a lot of that life gets into the water. The alpine lakes at 12,000 feet in the Mountain US are so pristine partly because there’s just very little nearby that can get into them.

  3. Douche Packer on February 27, 2018 at 10:11 am

    Thanks for a very reasonable write-up. One thing that got me fired up about Lincks article is that not everyone has the luxury of just showing up at the doc office for treatment for several different reasons. And while I have a very affordable health plan, I’m allergic to several antibiotics, so I can’t play around 🙁

    Does anyone know the cost of testing small samples of water for common backcountry pathogens? It’d be interesting for people to do this and share the results.

    • Lyle Gordon on February 27, 2018 at 12:10 pm

      I used Kar labs to test home tap water, they have a $50 test kit (basically just sample bottles you send back). For the cheap kit they only wanted tap way sources but if the lake/stream was free of sediment I suspect they wouldn’t care. However, it looks like they were bought out by Pace Analytical. You could inquire how much simple pathogen testing for water samples would cost.

  4. Jacob Portukalian on February 27, 2018 at 11:35 am

    I only filtered about 5% of the water I drank on the JMT last summer, mainly because I was too b̶u̶s̶y̶ ̶h̶i̶k̶i̶n̶g̶ lazy to filter it. I more or less used these exact criteria and didn’t experience any health issues.

    • Andrew Skurka on February 27, 2018 at 12:01 pm

      I’m selective around the JMT because it attracts so many people, many of whom are not experienced hikers and are not proficient with LNT practices. However, there are hundreds of side streams that flow across the JMT that originate in pristine basins, and I think those are reliably low-risk. But some lakes (e.g. Rae Lakes later in the summer) I would purify or bypass, and I would not trust creeks along which the JMT goes for many miles, unless I’m at the uppermost crossing/touch point.

      Keep another thing in mind: last year the Sierra snowpack was ridiculous, and that diluted everything more than it normally would have been.

      • Jacob Portukalian on February 27, 2018 at 12:09 pm

        Yup. Rae Lakes was one of the spots I filtered my water. Most water we got from the side streams or large “creeks” that would be extremely diluted if they had managed to get contaminated. I think it would be extremely difficult to contaminate the South Fork of the Kings River at the trail crossing for instance.

  5. Knut on February 27, 2018 at 12:05 pm

    For me, treating my drinking water is just like hedging my bets. There is no real reason not to do it, and plenty of downsides in the event the water is actually contaminated. No real reason to risk being ill for days of a trip when it can be avoided for at least minimised. I do agree with Linck that personal hygiene plays a bigger role in the game than treating water.

  6. Scott Drumm on February 27, 2018 at 7:42 pm

    Andrew – What I don’t understand – and why I disagreed with the guidance of your guides on this point during last year’s Sierra trips – is what’s the benefit of accepting the risk by not treating water in the backcountry? A Sawyer squeeze is incredibly fast and easy to use so it’s not a real time savings. Chemical treatments are generally taste and odor free.

    So from your perspective, what’s the upside of drinking untreated water that makes it worth the risk?

    • Edward on February 27, 2018 at 9:57 pm

      He is holding a Sawyer in the above listed photo and realistically, it’s the kind of information that can be used in selection of a water source even with filters or especially in the event of a filter failure.

    • BCap on February 28, 2018 at 6:21 am

      A blind taste test of Sawyer filtered spring water and ‘raw’ spring water is likely to get a random result. I’d argue strongly that the decision not to filter is primarily a philosophical decision and not a practical decision. For example D. Chenault recently wrote, “…I find the notion of tasting it in anything other than it’s original form insulting” (read his blog post for more details). I’m with you in that I don’t see the upside, but if someone feels that they can connect better with nature by drinking unfiltered water then that is their deal (so long as they practice good LNT and don’t start shedding cysts all over the place).

      • Scott Drumm on February 28, 2018 at 7:07 pm

        OK, so it’s basically like riding a motorcycle without a helmet. Maybe not the logical choice, but it’s a free country so… 🙂

        FWIW, I agree with the Edward too – it’s still good practice to follow Andrew’s tips to selecting water sites even if you are using a filter or other purification system.

      • Lyle on February 28, 2018 at 7:20 pm

        My thoughts are that the decisions should predominantly be about managing risk. While I may be slightly safer carrying bear spray in the Sierra, the chance of a black bear up there attacking me is quite low so I don’t bother. Sadly, we don’t really have good statistics on the prevalence of water borne pathogens in different sources (ex. high alpine springs vs. developed stock tanks in lower areas) to base the decision upon. There is also likely a different chance having symptoms and their severity for different people so you’d need to know your own response as well. With this lack of information I think the suggestions outlined here are a good starting point. If you are particularly risk-averse you could simply filter everything as its not particularly time consuming and the equipment is inexpensive and lightweight. On shorter trips you’ll probably be home by the time you show symptoms anyways. In the end you’ll probably die in a car crash before even getting to the trail.

    • Andrew Skurka on February 28, 2018 at 7:40 pm

      Purifying backcountry water might be analogous to buying car insurance.

      In this case, I have car insurance, because I always go into the backcountry with some type of purification method (usually Aquamira). That keeps me protected in a basic accident, i.e. it ensures that I can drink a nasty water source if it’s the only one around.

      But when considering my levels of coverage, I choose the minimums, figuring that the risk of needing $1,000,000 for bodily injury is probably not worth the marginal additional expense.

      I think you’re asking, understandably, what is that “marginal additional expense,” since purifying water is relatively easy to do. I admit that it’s small — but chemicals take a few minutes; pumping demands effort; and the Sawyer is awkwardly sized, has a relatively slow flow rate (compared to a standard bottle opening), and requires squeezing or rolling your bottle. I’m not opposed to doing it, but why would I buy those additional levels of insurance for incidents that I think are extremely low risk? We all have to make that decision.

      As BCap referenced, there is also the philosophical aspect of it. I like the sense of having an unfiltered (pun intended) experience in nature, by experiencing everything on its terms. This is the same reason I like to hike off-trail and to go solo — trails sometimes give me a museum-like sensation, and with groups I have the opportunity for emotional support if needed.

  7. Alan Richards on February 27, 2018 at 9:17 pm

    I have historically pretty much treated all my water. However, one time some years ago I was hiking high on the west side Rocky Mountain National Park, and came across a small brook (maybe a foot across and equally deep) that was babling its way down a moderately steep slope a little below timberline. It was cold and clear, and pretty remote – I dropped down and drank directly from it, without much fear. I didn’t know Andrew’s six questions then, but it met them. Very refreshing!

    But – some days after, I had intestinal distress and learned I had giardia. Lesson learned! – I’ve treated my water since, and haven’t made exceptions.

    Andrew, you’ve hiked many miles and apparently you have drunk a lot of untreated water – I wonder if you’ve developed a resistance to giardia somewhere along the line?

    • Andrew Skurka on February 28, 2018 at 7:47 pm

      My understanding is that you can develop an immunity to levels of pathogens. In sufficient quantity, your immune system will not help you. But in lower doses, it might. Those with weak or underdeveloped immune systems might want to think twice about drinking unpurified water.

      I’d be curious where your creek was. There’s a lot of people and elk up high.

      It’s possible, though less likely, that your suspected source was not it. Could you have been contaminated by a group member or by another water source that was not properly treated?

      • Alan Richards on February 28, 2018 at 8:57 pm

        Andrew, it was a long time ago, and I’m not sure exactly where it was – if I come across it in my notes, I’ll get back to you. I’m pretty sure it was that brook. It was just my wife and I; she didn’t drink from the brook, and we treated all other water.

        Of course there could have been elk, deer, sheep or beavers above there that I wasn’t aware of – but then, that would describe many of the places we like to hike!

  8. Dan W. on February 28, 2018 at 12:54 am

    Good info. Many thanks. I use a CNOC Vecto Water Container and a Renovo MUV modular water filter. The bladder has a large inlet for effecient water collection and a 28mm threaded outlet that screws onto a bottle or filter. MUV 2 will filter “100K Gal.”

  9. Joe on February 28, 2018 at 4:14 pm

    In almost 50 years of backcountry travel, I have almost always carried purification gear, and seldom used it.

    Never had issues.

    I’ve drunk some nasty goo in the desert when I was too thirsty to wait, too.

    Death Valley with burro shit in the water in ’74 comes to mind. I was so thirsty I would have drunk their urine straight. (I hadn’t had any need to pee for hours.)

    Otoh, on a winter-long MTB tour in Mexico in ’86-87, we iodined everything except beer. Never had a tasty drink of water that whole winter.

    In the Colorado high country, where I spend my time these days, I just mostly drink what I find when I’m thirsty.

    I wonder about the idea of gradually gaining immunity.

    Is that proven, or just supposed?

  10. folecr on February 28, 2018 at 8:07 pm

    I grew up in the tropics and vividly remember intestinal distress as a kid.

    I’ve drunk without filtering from sources in the High Sierra and not-so-high Sierra. Haven’t had issues. Outside the Sierra I’ve always filtered.

    Those six questions up there are solid. It might be useful to order them by priority… (6) and (2) are tops for me.

  11. Richard on March 2, 2018 at 9:56 pm

    Since I use a filter and carry it, it just makes sense for me to filter all my drinking water. It saves me from having to decide. Maybe if I used chemical treatment, it would matter.

    Also, some people are quite resistant to water borne pathogens and some people are not. And an individual’s immune system can change over time as well.

    Filtering water is easy. It doesn’t take much time or effort. I don’t lecture anyone about their practices and won’t listen to anyone telling me I am wasting time filtering.

  12. Ethan Linck on March 4, 2018 at 10:39 pm

    An excellent heuristic for minimizing risk when drinking untreated water — thanks for the post! All the more so for including a photo of the Napeequa 🙂

  13. Buck Nelson on March 6, 2018 at 5:38 pm

    Andrew, I think overall your article is sensible and well reasoned, but here are some things I see differently.

    I’ve gotten giardiasis three times after drinking from water sources that most non-treaters would have considered safe: once from a brook in the Sierra, once from a brook in the middle of nowhere in Alaska (true, deep wilderness,) once from a spring in a wilderness area of Idaho.

    In nearly all series of comments on articles like this there will be several cases of waterborne giardiasis reported. It’s not rare. Imagine how common it would be if water treatment wasn’t a standard practice.

    As far as I know 100% of backcountry waterborne giardiasis outbreaks have been traced to water.

    The minimum infectious dose for giardiasis is not 10 cysts. The best science says there’s about a 2% risk of infection after ingesting a single cyst. And we are unlikely to drink an average amount of giardia cysts per liter. Usually we’ll be drinking zero per liter, until our luck changes. “Pulse contamination” likely results in a disproportionate number of backcountry giardiasis cases.

    A mediocre water source properly treated is much safer than an apparently pristine source not treated at all. “I thought it would be safe” are the famous last words of many gastrointestinal illness sufferers.

    Voles carry giardia. Marmots carry cryptosporidium. The EPA’s “Giardia: Drinking Water Health Advisory” says “Cysts have been found all months of the year in surface waters from the Arctic to the tropics in even the most pristine of surface waters.” Some quotes: “In three pristine watersheds in Washington, Ongerth Giardia cysts in 43% of 222 samples” “Roach… found Giardia cysts in 32% of samples collected from pristine streams” “Hibler (1988) found Giardia cysts in 19% of springs…”

    This is a topic I know well, both from personal experience and a ridiculous amount of research. Can we usually get by drinking carefully chosen untreated water? Sure. Is there still a significant risk? Absolutely. Is that unquantifiable risk worth it? That’s purely subjective, and the answer is likely to be dependent largely on our personal experience and the experiences of those we happen to know.

    • Andrew Skurka on March 7, 2018 at 9:04 am

      Thanks for sharing your thoughts, Buck. You’re always very welcome here.

      I’m surprised by the contamination rate of springs, and that you also got sick from one. If it’s really spring, shouldn’t the pathogens have been filtered out by the soil before the water re-emerges?

      • Buck Neldon on March 7, 2018 at 1:11 pm

        Thank you Andrew.

        It would seem logical that springs would be safe, naturally filtered water, but testing shows that it often isn’t true. In rural areas a common source of waterborne illness comes from shallow wells where water hasn’t been filtered through enough soil or where surface water seeps in. There are often springs coming right out of the ground that ran into the ground a short distance uphill, or ran through porous rock a greater distance away.

        “A synoptic survey of 37 springs in 2013–2014 found that more than 90% failed one or more health-based drinking water standards. A more intensive follow-up study in 2014–2015 on ten of the 37 roadside springs found that they consistently failed drinking water standards throughout the year, including some presence of both Giardia and Cryptosporidium cysts.”

  14. Brett on March 8, 2018 at 1:46 pm

    About stagnant water…This might be too far out there for most people to be comfortable with but I learned from one of the world’s premier outdoor survivalist and primitive skills educators, David Holladay, that when deciding to drink stagnant water always look to see if there are bugs. If there are bugs in the water that is a good sign that the water is ok because there is life in the water. If there are no bugs there is no life in the water and there is the reason for it that so don’t drink it. Like I said most people will not be comfortable with that but if you end up in a desperate situation with no way to treat your water this “suggestion” might save your life.

    • Andrew Skurka on March 8, 2018 at 3:07 pm

      I’ve heard that as well, but I’m not convinced that it’s true.

      First, if the water is safe enough for bugs, it probably also means that it’s safe enough for pathogens, too. It’s not as if paddle bugs eat up all the giardia cysts. Remember, most of the pathogens that will give you the runs (or worse) are not visible to the naked eye.

      Second, I have drank unpurified water on multiple occasions from seasonal/temporary potholes, like in rock-lined canyon bottoms. There were no bugs because there usually isn’t enough water to sustain life, not because the water is unsafe. Yet the water is probably safe to drink, but it gets blasted by 12 hours of sunshine per day and it’s only 6 inches deep, so everthing in there gets zapped.

    • Scott Drumm on March 8, 2018 at 4:21 pm

      The only way I could see that making sense is if you were concerned about chemical contamination, heavy metals, radiation, etc. But if you’re backpacking in a zone with those kinds of risks, you’ve got bigger issues to worry about! 😉

      • Bob S. on March 14, 2018 at 10:27 am

        Scott, if you’re into chemical contamination, heavy metals, and radiation you would probably find “The Meadowlands: Wilderness Adventures at the Edge of a City” a fascinating read 😉

        I got food poisoning sampling local cuisine traveling in a rural part of the world 2 years ago and I can say with confidence that what doesn’t kill you makes you wish you were dead. I’m more inclined to filter everything I drink now and I won’t eat food I can’t identify – I’m looking forward to Part II of the series.

  15. James Kerr on March 22, 2018 at 2:44 pm

    Hi Andrew,

    I just ran across this article from the Journal of the Wilderness Medical Society, on evidence-based medicine and wilderness water safety:

    It makes the case that there is little evidence that the great majority of wilderness water sources present significant risk of infection. It also makes the point that in focusing on educating backcountry travelers on water filtering/purification, that we fail to put focus on the much greater risk of hand-to-mouth contamination. We should be stressing personal hygiene in the backcountry. Quoting the article; “It must be impressed upon backpackers (just as it is impressed upon health care, food industry, and daycare workers) that stopping hand-to-mouth spread is the key to preventing gastrointestinal infection. Diluting this message with unfounded concerns about wilderness water quality or the relative merits of various water-treatment methods serves no useful purpose.”

    All that said, I’ll probably continue to filter in most situations.


    • Andrew Skurka on March 22, 2018 at 2:46 pm

      Washing your hands is extremely important, but it’s not mutually exclusive with being water smart, too.

    • Buck Nelson on March 22, 2018 at 3:53 pm

      With no disrespect to you, Mr. Kerr, but I consider Dr. Welch to be a crackpot on this topic.

      He is someone writing outside his field of expertise and I don’t know of any experts in the field (epidemiology) that agree with him.

      Dr. Welch makes numerous demonstrably false statements in that editorial. First of all, there is mountains of evidence that many people get sick from drinking backcountry water:

      Secondly, there is no evidence that hand-washing is MORE important in the backcountry. It’s a made up fact. Dr. Welch has no evidence to support it. I know this because he provides no evidence in his writings, and has been unable to provide me with any evidence when we’ve corresponded. The only scientific study I’ve seen (Medical risks of wilderness hiking) that compared water treatment to hand washing found water treatment to be much more important. That paper is quoted and linked at the above link.

      Clean water and clean hands are always a good idea, regardless.

  16. James Kerr on March 23, 2018 at 11:32 am

    Thanks, Buck, for the link. A wealth of information in that post (now bookmarked for future reference).

    I suppose hat Dr. Welch argument re: hygiene resonated with me because the only time I have ever experienced any gastrointestinal distress in the backcountry (in 50 years of extensive wilderness travel) was on an extended climbing expedition in the Alaska Range. As all of our water on that 80-day expedition was from melted glacier snows, we attributed our outbreak to poor personal hygiene and dish-washing.

    Since that rather dreadful experience (diarrhea at 20 below ain’t no fun!), I have been a fanatic at maintaining personal hygiene, and have not been stricken since. Of course, I’ve also filtered 95% of the water I’ve consumed in the backcountry since that time.

  17. Taylor on September 7, 2019 at 7:22 pm

    There is a small lake I like to go to, at the base of a mountain at about 9000ft, it’s covered in snow most of the year but by the end of summer usually you can see the crystal clear water and much of the ice melts away. There is a stream that comes out of that lake that looks like the most picturesque thing I can imagine, postcard worthy for sure, cold and crystal clear. It tastes as good as it looks. There also happens to be a small herd of elk that got caught in an avalanche and are buried in the snow/ice right at the edge of the lake you can’t see from the stream. If you go in September you will see bloated carcasses in the water. If you go in June they will be totally concealed by snow. Obviously that a rare occurrence but it’s something to think about.

  18. Allan on October 15, 2019 at 4:58 pm

    Everybody gets GI complaints every few years or whatever. Most of these people never backpack. But backpackers with GI complaints always blame the water.

    If reader X says he or she did Y and got sick/didn’t get sick, it’s not useful information. Similarly, if my uncle smoked cigarettes, and lived to 95, it’s not a significant statement about public health. And if I said I saw a UFO, and that proves the aliens are here, that’s not meaningful.

    I’ve mostly NOT treated water in my decades of weekend backpacking in southern New York and New England. In recent years, I have. Mostly, I just don’t want to drink dead spiders and whatever else. And available methods have been greatly improved.

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