The clear running mountain stream is an iconic image of purity. Think of every advertisement for bottled water you've ever seen. So naturally when our heroes are hiking through the wilderness and they come across a babbling brook, it's time to dive right in and drink their fill without a second's thought right after filling the canteens.
Those of you who are regular campers or have attended any form of survival training know where this is going.
In Fictionland, water that's unsafe to drink will look unsafe to drink. It will be brackish. It will smell bad. There will be dead fish floating on top. Maybe a cigarette butt or used condom if it's in a populated area. So if that mountain stream is crystal clear and Bambi and his pals are all drinking happily from it, then no problem.
Except, of course, for the myriad waterborne parasites and diseases that sicken and kill thousands in the real world. Animals have resistances that humans long removed from the wild don't (and they get sick fairly often anyhow). There's a reason water purification kits are standard issue for hikers, campers and survival kits worldwide. Not to mention the billions of dollars governments pour into building and maintaining municipal water facilities and sewage treatment plants.
Also, in some deserts, any water that is beautifully clear and fresh looking (i.e., nothing lives there) may be caused by natural arsenic deposits that turn up in some rocky areas. Water that looks green and mossy and full of gunk will give you dysentery at worst, but drinking the perfectly clear death water will end your hike for good. In the mountain West of the US or other areas where there is mining activity, cyanide and cyanide-based chemicals can have the same effect, as can pesticide runoff in heavily farmed/agricultural areas. In otherdeserts, water that is perfectly clear with nothing in it and isn't running may well be irradiated. Different means, same ends. Or perhaps different ends...
This trope is often found in historical fiction, where characters will be shown drinking and bathing in fresh water without a worry or concern. This is especially true of the Purity Sue, who will imply her "specialness" by the amount of time she spends bathing. Time travelers might also take the opportunity to sneer at the filthy beasts they meet who are simply too stupid to keep themselves clean and too debauched to drink cool, clear water instead of alcohol. All nonsense, of course: people in the old days didn't know that the diseases associated with drinking or bathing in plain water (cholera, dysentery, typhoid, polio, etc.) were caused by waterborne pathogens, but they did know that both activities were risky, and boiling that much water was simply too damn expensive for many. (And the reason many people of historical times preferred alcohol or tea to pure water was that alcohol and/or the process of making beer or wine or boiling for tea were some of the earliest forms of water purification. Similarly, moss was one of the earliest water filters - so mossy water was often safer than clear) Some writers take the trope to eleven and give their Purity Sue characters hot and cold running water and showers. In Norman castles. We'd like to claim that this is more common in fan fiction, but sadly it's a common mistake in professionally published fiction.
It's best to assume that untreated water is unsafe to drink, particularly if it's passed a road, a village, or a forest. There are streams that have been filtered by mineral layers (more in some countries than in others), but generally they shouldn't be trusted without confirmation, preferably from a local who has drunk from it for many years. Even then, those people may have simply built up certain immunities. Even the purest-seeming mountain stream may be tainted with parasites like Giardia lamblia, also known in Western Canada as "beaver fever". No, not even the First World is immune.
Also see I Ate What?.
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Anime and Manga
Subverted on the anime Meiken Jolie (aka: Belle and Sebastian), the boy Sebastian drinks from an apparently crystal clear stream despite the warnings and misgivings of his dog Belle (he also failed to notice a symbol carved on a rock nearby that warned people against drinking from such stream), and he falls terribly ill making Belle to go great lengths to protect him and causing her to get captured.
Averted in an episode of Machine Robo Rescue. When Ace, Taiyou, and the Victim of the Week get lost in the jungle, Ace makes a point of putting the water they find through a makeshift purifier, noting the potential dangers of drinking "natural" water.
InuYasha: Played straight. Not only do the characters native to time period freely top up every time they encounter a pool of fresh water, but even the girl from the future (whose modern living would likely make her much more prone to illness) does so. On top of this, the story is set during the feudal era where death, disease and rotting bodies on battlefields are commonplace. And that's before we get to the Youkai that live in, and use, pools of water as well. The only time we ever see a character avoid drinking water from a pond, it's because there's a dangerous Youkai living there, not because the water itself might not be safe to drink.
In one storyline in Love Hina, Keitaro, Naru, and Nyamo are wandering a desert. Just when all hope seems lost, they find a lush oasis with a waterfall and lake. All three quickly slake their thirst. It's later subverted when Naru suffers a bad bout of diarrhea because of it and risks her and Keitaro's chances of sending their applications to Tokyo University on time just to keep this from Keitaro. Keitaro later points out that her condition is nothing to be ashamed of and even points out that it might have been caused by something in the water. One of the more justified cases, since they were close to dying of thirst at the time.
The trope is subverted in the Marvel war comic The 'Nam where the lead character on his first patrol in the bush of Vietnam is about to drink from a river and his experienced comrade stops and shows what he has to do to properly treat the water to make it fit for drinking.
Alison Bechdel's auto-bio comic Fun Home mentions a river near her family's house whose sparkling beauty is due to pollution preventing any life in it. Similarly, sunsets at home were much more impressive before the clean air act.
A commercial for Brita water filters used this trope completely straight. We see a wonderful mountain stream in the woods, while the voice over says "If we lived here, we wouldn't have to worry about water quality..." Cut to a bustling freeway, and the narrator continues, "Unfortunately, most of us live here." Yes, because water running straight through underground pipes from state-of-the-art water treatment facilities to your apartment is SO susceptible to air pollution.
Another water filter commercial subverts this trope; it starts as a fake commercial for bottled mountain stream water, with a rugged outdoorsman waxing poetic about the "cool, clear, pure mountain stream" he's walking alongside... until he happens upon a pair of mountain men straight out of Deliverance washing their long johns in the "pure mountain stream".
Yet another commercial (for bottled water) has a guy in a bear costume standing in front of a stream saying "me? drink from that stream? are you nuts? do you have any idea what animals do in that stream?"
Subverted in an old Sprite commercial where the spokesman goes to a stream to see what the fuss for water is all about. He sees a bear walk into the stream, squat, and... "Whoa! I thought y'all did that in the woods!"
A billboard for a beer attempted to invoke this by comparing their product to glacial meltwater. Unlike melting snow, however, glacial meltwater has a great deal of powdered rock suspended in it, making for a substance that is bitterly cold, foul-tasting, milky white, and a powerful laxative. Hardly the "clear, cool, and refreshing" they were going for.
Parodied in the Jackie Chan film The Tuxedo. The film opens with a shot of a pure mountain stream, and then shows a deer urinating in it. We then follow the water downstream where it is bottled by a water company.
Lends a whole new meaning to the Deer Park logo.
An earlier Jackie Chan film shows Jackie kneeling to drink from a sparkling stream...and then he notices the boy upstream a few yards peeing in it.
Subverted in Rashomon. A notorious bandit is finally captured when he takes a drink from a river and becomes violently ill. He explains "A snake must have died upriver."
Averted in the TV movie Kidz in the Wood. At one point two characters stuck in a scorching valley stumble upon a pond, and one of them suggests they get a drink. The other one says not to, noticing the bleached white steer skull laying right next to it.
In the 2008 film Australia, the cattle drivers attempt to stop at a river to refill their water and let the herd of cattle they are bringing to the city of Darwin have a drink of water. Three or more die at the river, which is tinted off-color. As one of the drovers says to the leads, "The water's no good, boss." Of course, this doesn't stop them from freely drinking out of public fountains and streams that look clear.
The early western film Hell's Heroes (later remade as 3 Godfathers) has this as a major plot-point; the only water in a particularly ferocious stretch of desert looks OK, but as noted above it's laced with arsenic. (A sign is posted warning this.) In the end the last surviving protagonist drinks the water anyway, because it's the only way to stay alive long enough to get the baby he's trying to save to the nearest town.
Averted in Cabin Fever. It sure looks nice, but if you drink the water in this town, you're dead. Just like in Mexico.
Barbossa made specific mention to the water being fouled by the body. The bad water didn't cause the guy's death, the guy's death caused the bad water. The hope was that he hadn't been there long enough for it to become foul.
Parodied in The Scorpion King. When the heroes find an oasis, Cassandra asks if the water is safe. Out of nowhere, the Plucky Comic Relief sees the water and jumps in, cannonball-style. Mathias quips, "Not anymore".
In The Way Back, the characters spend most of their time trekking through the wilderness, and drink unpurified water all the time with no ill effects. Semi-justified in that they're far away from any man-made pollutants, and since most of them came from a gulag, it was probably much less contaminated than what they were used to drinking.
Dances with Wolves has a scene of John Dunbar going to fetch a bucket of water from a nearby lake for the first time, but he stops just as he sees the corpses of some deer in and around the area. He spends several days cleaning it up as best he can by himself, and he even notes that he took pains to see if they had been poisoned.
In all the chaos of the final action sequence of Casino Royale, it's easy to overlook that the water is as clear as in a swimming pool, when in reality, given that a building just collapsed into it, it should be murky.
There's a story about a king who went on a hunting trip with his hunting falcon. Separated from the rest of his party, he managed to find a spot where water dripped from the rocks. He collected a cup of it, but just when he went to drink, the falcon shook his wings and knocked it from the king's hand. When this happens for the third time, the king, enraged, kills his falcon; just then, his retainers find him. Not having the patience to sit through another slow collection, the king sends one of his retainers up to collect water from upstream - but the man finds a dead serpent in the water, and comes down to offer the king water from a canteen. (Aesop says: Avoid rash action, since you probably don't see the whole picture.) Of course, this trope applies all too well: The water was assumed pure until an actual dead thing was found in it, and had that thing not been there, it is probable that harm would have befallen the drinkers.
There is a version in the Arabian Nights where the water was dripping from a tree, and when the retainer went to collect it he found out that the "water" was actually venom and the tree was full of live vipers. Snake venom is actually not toxic to drink, but you certainly wouldn't want to drink it thinking it was water. (Or get that close to a tree full of live vipers.)
You don't want to drink snake venom if you, say, have a stomach ulcer. Although the snake venom being poisonous only in the bloodstream is a relatively new scientific discovery.
In The Girl Who Loved Tom Gordon, Trisha drinks some Cool Clear Water when she runs out of her bottled water. It causes a severe bout of vomiting and diarrhea, although she luckily manages to get over it relatively quickly. Less fortunately, that appears to be because it weakened her system enough that she got a worse illness, and her body just gave up on purging the toxins.
One The Magic School Bus book dealing with the water system specifically pointed out that a clear-looking glass of water might still have germs and other yucky stuff in it, hence the need for filtration. This includes the MSB class themselves, who have to get out and walk around the filters.
Partially subverted in the Lord of the Rings novels, when Frodo and Sam are entering Mordor. Sam comments that they will not find water to drink since orcs probably drink poison as water. Frodo responds that as foul as orcs are, they surely cannot live on poison, so he fills their water packs with the brownish water the orcs drink. Although it does taste foul and sicken them a little, it also helps them on their way.
Fully subverted earlier in Moria: Gandalf says that while there are water streams in Moria, some of them clean, this water is not safe for drinking, so they have to conserve water supply.
A beautiful subversion (of course) in Discworld: an explorer has spent his life (ironically) searching uncharted jungles for the Fountain of Youth, only to drink from it and die from water contamination.
Also a footnote in Reaper Man notes that animals living in water is no guarantee that the water is safe to drink, since they probably don't get out to go to the bathroom.
In another subversion, Ponder Stibbons once shows the Archchancellor some water from the river Ankh under a microscope to show how much stuff is living in it. Ridcully declares that anything supporting that much life must be healthy.
Despite the fact that you can draw a chalk outline on the river Ankh, many of the city's residents are convinced it must be safe to drink because it's passed through so many kidneys. There are fish that have evolved to survive in the Ankh, but they tend to explode when exposed to fresh water or air.
Used very sadly in the The Edge Chronicles as Rook discovers when he meets Twig and discovers that his plan to save his crew at River-rise failed because a crew member got water from a still pond, killing everyone except Twig, who was sick for a week and couldn't even clear the bodies from his ship.
In the Clive Cussler novel The Wrecker, a temperance group tries to demonstrate how dangerous alcohol is by showing how a few drops of alcohol can kill all the various microbes living in a small container of water. The audience, seeing how many things were living in their well water, resolve to never touch it again.
Averted in The Hunger Games, Katniss always fills her water bottle, puts purifying drops in it, and waits half an hour before drinking it. Even when she was about to die from thirst.
Played straight in Redwall, where characters frequently drink from streams, and the trope name is outright used for description. Semi-justified in that the characters are anthropomorphic animals.
Both subverted and played straight in Running Out of Time.Jessie, feeling thirsty after escaping from her fake 1840's village, stops to take a drink from water flowing through a ditch. An environmentalist stops her in time, telling her that despite the water's pristine appearance, it's nowhere near safe to drink. However, this also implies that Jessie's been able to drink from rivers back home with no ill effect.
Averted in The Stand, where no one would drink water without boiling it for most of the book, either while they were camping in the woods or after they hit their destination due to the sanitation systems being out of commission along with everything else after the plague. The only thing attempted with the stream while Stu's party is camping is fishing.
The Freeway Warrior adventure books acknowledge the need for drinking safe water. Part of Cal Phoenix's Fieldcraft skill training involves identification of safe water sources, Water Purification Tablets can be found, and Medkit supplies are often stated by the text to contain more water purification tablets. On top of that, the primary sources of water in the books are from water tanks and the surviving colonist's water supplies.
Live Action TV
An episode of Monk had friends managing to convince a dehydrated Monk to drink from a stream. For once, Monk's paranoia may have been justified.
That being said, he'd gain nothing from dying of dehydration because he was afraid of catching a waterborne disease. Kind of a damned if you do, damned if you don't situation.
Played with in the Disney version of Davy Crockett. On the way to the Texas, Davy's and his companion George Russell come across a brackish pond with what looks like white algae growing on it. Crockett says to Russell that the water looks suspect, but Russell states "I don't care, it's WET."
LOST plays this straight as a die. Running out of bottled water? Find the nearest available cavern with water in it. Totally safe.
As the water on that specific Island turned out to have magical healing powers, it may be partially justified.
Averted in Lois and Clark. Clark and Lois ended up stranded on a deserted island and Lois was about to drink from a pond. Clark used his microscopic vision on the water and saw that it had bacteria. He suggested that Lois try to drink somewhere else.
The pond had been deliberately contaminated by the Villain of the Week, and Clark "finds" a better source of water by punching a hole in a nearby rock to produce an instant spring.
An episode of the new Hawaii Five-0 has an escaped convict with a couple of hostages stop to drink from a stream in a Hawaiian rain forest. Shortly after McGarrett and Chin come across the same stream, and McGarrett starts to take a drink, only to have Chin stop him and tell him the water is infested with parasites, and will make him very sick, very quickly. Cut to convict and hostages... Normally it would be completely out of character for McGarrett, a Navy SEAL who grew up on the Island, to make such a rookie mistake. It can be explained by the writers having to rewrite the episode to include Scott Cann's real life leg injury. Since Danno couldn't have helped chase down the escapee and hostages using a cane in the rain forest, Chin accompanied McGarrett. The writers had to explain the sudden illness of the escapee and hostages, so that was left in with Chin, correcting McGarrett.
Explained in an episode of MythBusters: during Duct Tape Island, Adam and Jamie try to find water and Jamie tells Adam that the first water source they find is actually stagnant and isn't usable. Adam follows one of the safer examples of the trope later when he finds a running waterfall that goes over a large moss bed into a stream, but chooses to make the effort to collect the falling water instead of the water that's at the lower level of the stream where he is.
Being based off of Real Life (sort of), this pops up in Survivor - finding water is one of the most important bits, and usually they're Genre Savvy enough to know not to drink stagnant water.
Played straight in the Marty Robbins song "Five Brothers" which contains the line: "When first they saw the killer he was by a water hole, five rifles rang out through the night and killed the gambler cold/ The desert is their keeper now for this a traveler said, the poison lived within the well now six of them are dead"
Averted in Fallout, where a survival manual directly states that in this high-radiation, post-nuclear setting, clear water—especially if it's not running water—is most likely radioactive and should be avoided. When you consider the background radiation is high enough to produce irradiated rats the size of a man and irradiated scorpions that are bigger, this is probably good advice. Fallout 3 goes out of its way to warn you of this fact, as short of the major rivers, virtually any outdoor water pool has rising brown steam. Most still water sources like toilets and sinks are irradiated and there's even cults that consider irradiated water to be holy.
There are no pure water sources anywhere in the game until after you complete the main storyline and begin the Broken Steel DLC. Not even the water flowing below the aptly named Oasis, which is a healthy and strong forest growing among all the dead wastelands, is pure. Everything you touch or drink (save for purified water bottles) will irradiate you.
However drinking irradiated Water isn't really that bad. While it will increase your Radiation count it will typically only increase it by 2 or 3 Rads per drink, where you need at least 200 rads to have any side effects and can take up to 1,000 rads before you die. Meanwhile water recovers HP by 20 a drink and Rads can be removed by commonly found items. However there are rare extremely contaminated Water supplies that will only restore one or two HP while giving you 15-20 Rads a drink. These are generally bad to use.
Deconstructed in New Vegas. You can drink the water straight out of the Colorado River, which is clean and not irradiated, with no ill effects. This, along with the Hoover Dam, makes the New Vegas stretch of the Colorado a natural resource the NCR and the Legion are willing to spend nearly any amount of money and lives to control.
Played with in Mother 3: there are hot springs scattered all over the world which automatically restore all Hit Points and Mana. There is one, however, that the party encounters while tripping on wild mushrooms that turns out to be a garbage-filled cesspool if they revisit it sober. Weird thing is, when your party thinks it's a hot spring, it functions just like a regular spring, making this a Clap Your Hands If You Believe variant on this trope.
Of course, while you're tripping balls, Boney will stay clear of the "hot spring" altogether, on top of regularly barking when you approach the "people". Reason? Boney didn't eat the 'shrooms.
In chapter three, Fassad and Salsa can visit a desert oasis guarded by a dung beetle (appropriately named Wan Sum Dung.) When the water is first approached, it's sparkling with freshness and heals Salsa up. After giving the beetle some dung... the water still sparkles, though it now smells "dung-like".
In Lost in Blue, at the beginning of the game, there's a freshwater river that has fish swimming in it, and your character comments that it looks safe to drink. There's also a small hole in a temple you have to go through that is apparently safe to drink from.
The Dark Cloud and Dark Chronicle games feature Springs in the Randomly Generated Levels which can both quench the character's thirst and replenish their HP. These springs can occur in the Sewers, in the middle of a Forest, in underground grottoes, in cliffside caves, in volcanic hot springs, and in palace fountains. At least the ones in volcanoes are boiled... never mind the acidity.
Though one doesn't get to do this, in The Oregon Trail series from II and on has a woman warn not to dig a waterhole, commenting her husband died because of Cholera from it. Also the draft animals often get ill from still ponds due to Alkali Poisoning - often stating if one must drink the water, it should be moving water.
And your party members can also get "water poisoning" as well as "alkali sickness".
For a guy who lives in a desert, the Prince has absolutely no qualms about drinking out of ANY water supply in Prince of Persia: The Sands of Time. (Semi-justified in that most of the time, he's drinking out of fountains, but still.)
In Prince of Persia: Warrior Within, he's drinking from fountains again, except some of those fountains are in a castle that has been abandoned for an extended period and the stone has grown old, broken down, and covered with moss and plants. Admittedly, the water is clear and running, and the castle does turn out to have an aquifer.
Prince of Persia: The Two Thrones changes it to just about any body of water, but still mostly fountains. Of course, being set in Babylon, just about all the water needs to be drinkable, and the Prince drinks from places he can reasonably assume has clean water, especially since he claims to have extensive knowledge of the city.
In Dwarf Fortress, water which you can't see anything in is drinkable, but "murky" water, generally found in places without running water, isn't.
DF is still rather odd about this (as of 31.18). One level of standing water (or 7/7 units) will be stagnant and spread mud and can't be filtered any number of pumps, but a well over it will yield pure water. Any water level 8/7 or higher will also be pure. Also, toxins won't spread from their tile, so the corpse of a poison-bleeding creature will poison a tile for at least a game year, but the water flowing from it will still be safe.
Subverted in that water, while drinkable, is not an appropriate beverage for dwarves. Water is for all sorts of neat tricks, but dwarves who drink water become unhappy because it does not contain alcohol. It is a wonder that this game allows dwarves to even think of manufacturing waterskins that cannot be filled with alcoholic drink... a problem the SyrupleafLet's Play ran into, incidentally. This has since been rectified, though you can still order your soldiers to only carry water if you really want to.
Incidentally, polluted water won't actually harm your dwarves if they drink it, just cause an additional morale penalty. Washing an injured dwarf's wound out with it is another matter And Armok help you if you get vampire blood in it!
Averted in the Atelier Series. You can find Fresh Water everywhere... but you only get it to a usable Pure Water state (which can, among other uses, be used for food or consumed for some hit points) if processed.
Alluded to in Golden Sun; as he's leaving home Garet's siblings remind him not to drink from stagnant ponds, to which he replies indignantly that he knows that already. Subverted in that the only sources for water your characters actually drink (the oases in the Lamakan Desert are apparently just for frolicking in) are the Fountain of Hermes in the first game and the Lemurian fountain in the second, both of which explicitly have healing properties and are purified by Alchemy.
Averted in the mod for Battlezone 2, Forgotten Enemies, one of the planets, Spartacus has trace amounts of arsenic in it's water despite looking clear in the missions and actually, has been wearing down on the filters that the EDF relied on. Corber mentions in one of the monologues that one of his fellow infantry men drank some of the unpurified water and died about an hour later.
To get through the desert in King's Quest V, Graham will have to drink straight from the various oases scattered across the landscape (and can also get a drink from an abandoned well in the same area). The narrator comments "Ah! Life-giving water! Nectar of the gods!"
The Wandering Ones averts this trope as well, since the writer is knowledgeable about wilderness survival, and puts his research into the strip.
From Concerned; this joke: "Don't drink the water. It's $2.95 a bottle." , and the joke from the comic's banner ad: "Don't drink the water. I forget why..."
Directly averted in an episode of Captain Planet. Kwame at one point drinks a bottle full of river water, unaware Wheeler didn't boil the water thoroughly. Sure enough, the water is bad (Dr. Blight and Sly Sludge were operating a sewage dumping operation just upstream) and the other Planeteers have to go on a "Fantastic Voyage" Plot to blast the parasites from inside his body. The episode had an And Knowing Is Half the Battle at the end addressing this and teaching viewers how to purify water.
Despite her love for camping, Sam of Danny Phantom apparently doesn't know or follow this trope; as she (and Danny) easily fill their bottles with water from a lake they found.
An aversion, along with a Getting Crap Past the Radar version of the page quote, find their way into Rocko's Modern Life. The gang goes on a camping trip and Filburt refuses to drink from the stream water because, "fish are dating in it."
Subverted and Played for Laughs in Futurama, where in an episode where everyone is camping in the woods, Leela wakes up and starts the morning by heading to the river to get a drink. While it seems like she's mentioning how there's nothing more refreshing than the fresh flowing water from the river, she reaches into the water to pull out a 6-pack of Canadian beer.
The WC Fields quote is delivered by Reggie Thistleton in a flashback from Woodhouse on Archer.
During the The American Civil War, the civilian Sanitary Commission had a nightmare of a time trying to get volunteer units (which made up the bulk of the Union Army) to get their drinking water upstream of the latrines.
People have not been drinking pure water for very long. Up until the late 19th century, the favorite drink of the American people was "soft beer" or water with 1-2% ethanol as a disinfectant. The major part of the purification process when making beer, however, does not come from the alcohol, but from the first stage in brewing, which is boiling water. On a side note, most rural people who drink water nowadays draw from an aquifer, which is always cool, clear water albeit minerally.
For this reason, beer has been the staple drink of the populace since literally the beginning of civilization..... one of the oldest clay tablets ever found in Mesopotamia was a recipe for beer. To reiterate: it was one of the very first things people considered worth writing down, EVER.
A more recent example of beer being safer than untreated water (which in fact was documented as proof of it) occurred during the Broad Street Pump Cholera Outbreak in the 19th century, where the local brewery managed to avoid losing any workers to the disease due to the fact that their workers got an allowance of free beer as a perk and so didn't drink any water. To complete the circle, there's now a Pub close to the site of the old pump, named the John Snow◊ in honour of the Doctor who identified the pump as being the source of the outbreak.
While not all cultures relied on alcohol to make drinking water safe, those that didn't generally had another method, China's tea obsession being a good example since boiling kills most things, plus tea is an antiseptic. There's also a theory that a contributing factor of humans starting to consume milk from other animals was the fact that it was a hell of a lot safer than river water.
Tea, and even more, coffee, were what Muslims drank, alcohol being forbidden (although only-slightly-alcoholic beverages were often permitted, as well).
There was a small town in Chile that one day got their tap water system, with an official celebration and everything. The very next day, half of the town was sick, as they stomachs weren't used to that water, having always drunk water from a well or from the river. Also, it's a common recommendation when traveling to another country to never drink their tap water, just stick to bottled water or other drinks, so you can be sure nothing bad will happen.
New York City's water supply system uses no active treatment. Protected natural watersheds supplying well designed reservoirs allow the water to flow directly from the collection point to the tap without any additional intervention such as filtration or chlorination. Most consumer problems with NYC Water is a result of the "last mile" pipes in the local buildings, which are of uneven quality.
Almost all of New York City's water comes from the Catskills (the large reservoirs on the east side of the Hudson are supplemental and mainly serve as holding areas for the Catskill water). Water from montane springs or streams above any human or beaver activity there is generally drinkable without any filtration, and delicious to boot. The 19th-century naturalist John Burroughs, who grew up in that region, once said that you could survive on Catskill water alone for a few days; it was that good. He wasn't exaggerating.
Similarly the city of Seattle takes most of its water from the Cedar River, whose watershed is fenced off from any human encroachment (the fence around the watershed encloses more land area than Seattle proper). The Cedar River system is one of a handful of water supplies certified to operate without filtration.
The existence of moss often means that a water supply it's in is slightly safer than one it's not. While moss is a primitive water filter at best, it will filter some of the worst contaminants (specifically visible worms such as roundworms and hookworms), and its presence as living plant material generally means the absence of the most immediately lethal poisons or extreme radioactivity.
In the East Carpathians (any maybe in other parts of it too) it is generally safe to drink any water which hasn't yet run through a pasture or other agriculturally used area. Some fountains contain a lot of sulfur and other minerals, some even have a dark rusty color, strong smell and funny taste, they are still very healthy.
While lots of water in contact with volcanic activity (not necessarily an actual volcano) tend to have a bit too much sulfuric compounds to be safe to drink, lots are actually ok to drink and may be safer than other water in the area, since its content can limit the species able to survive there. However, this is not something to take as a good pointer towards possibly safe water - the safe ones are probably much more rare than the really, really dangerous ones.
The Ozark mountains are full of all kinds of stuff that can help the wise and hinder the unproven. Water flowing over 'mossy' waterfall, sometimes flowing down clumps of moss hanging 20 feet or more, is perfectly safe to drink, no matter what's up river. The river water, on the other hand, is full of leeches and rotting leaves. The well and spring water is most interesting, as it has enough sulfur to be nasty tasting, but not enough to harm you. It just gives you terrible B.O. and makes you unattractive to biting insects (and most humans) after about 3 days of regular consumption.
Most parasites that animals get are water borne, like giardia which infects 98% of whitetail deer.
Subverted in extreme survival situations, when no alternative water source is available. Thirst can kill you even faster than contaminants, and most water-borne ailments are treatable if you make it back to civilization. The general rule is that you should only drink unfiltered water if you are absolutely going to die without it.
With still water in nature, cloudy with algae is usually much safer, particularly in deserts. If there's water that isn't moving and doesn't look like it has life in it, there's usually some poisonous reason why, particularly in regions where most life latches onto any available moisture.
Many ancient cultures like Rome, Greece, and Egypt knew about water purification and writings explaining various methods have been found dating back to 2000 BC (sand and gravel filtration, boiling, straining, building massive aqueducts, etc.). Several ancient civilizations even had running water (Rome, the Minoans, etc.) and bathed regularly.
The city of Bath in the UK was founded on a number of hot springs nearby, but even bathing was prohibited for nearly 30 years after one of the springs was found to contain a water-borne parasite.
Entirely justified, as plenty of parasites can be contracted via skin contact, not just ingestion.
Spelunkers exploring deep into "living" limestone caves often find that the water in underground pools is as pure and drinkable as it gets, provided bats haven't been roosting over it.
Similar to the cave entry above, the cenotes of South America (limestone sinkholes) actually do follow this trope. Since the limestone naturally filters the groundwater before it reaches the surface, it is remarkably clear and pure.
For extra fun, if you've been doing this, see if you can find out what neat and interesting things microbiologists doing work on the local water have found. Wildlife waste can make it particularly nifty!
Of course, cenotes only follow this trope if there's no drought to significantly lower the water level. If it's too low, thirsty animals are going to jump into them and drown if they can't get out. Guess what happens to the water, then?
Near the Dainichi Temple of Japan, there is a holy spring with extremely clear water. This clear water contains arsenic! It's the reason why the sokushinbutsu, the self-mummifying (tantric/shingon-shu) Buddhists of the Dainichi Temple were so extremely successful with their mummification - unlike other locations - because arsenic can't be degraded after death, so the corpses stay free of parasites and insects.
In addition, arsenic (and various other heavy metals, including silver, mercury, and lead) are antibacterial, which would have helped the mummification process by killing off decomposition-causing microorganisms.
Many "ultralight" campers and hikers will drink water from mountain rivers and streams, forgoing the weight and inconvenience of filters and purification chemicals. Whilst it is true that most waterborne pollution and disease comes from human habitation and agriculture (especially the nastiest kinds, heavy metals and viruses that can't be simply filtered out), even small fast flowing streams in high mountains are not guaranteed to be safe. The odds of picking up something nasty are low enough that the trend persists.
Played straight by hobby aquariums. Since algae lives on the waste fish produce, if the aquarium is full of algae, it’s a sign that the tank is overstocked and the fish won’t last much longer.
Inverted in that sometimes algae growth helps to absorb the waste, protecting the fish. In severe cases, removing the algae without solving the root problem can cause the waste levels to spike and crash the tank. Some reef aquarists even use macroalgae as a secondary (or even primary) filter.
Averted in Rome—there are fountains all over the place, and even the ones that don't look like drinking fountains are perfectly safe to drink.
Also averted in Iceland. If you're not in an area with lots of geothermal activity, it's perfectly safe to drink directly from the rivers and lakes.
Most of the lakes in the Austrian and Swiss Alps have drinking water quality, and large regions of the countries draw their drinking water from alpine springs.
Zigzagged in Sweden where in the fjällen-areas (the mountain areas in the northwest of Sweden) the clear water is most likely safe to drink. In other places you should not drink from or even swim in clear lakes since those lakes are often sour. Healthy lakes are instead often dark/brown due to humus particles from the surrounding woods (humus-particles can't be solved in sour water and just sinks to the bottom).
This trope is one of the reasons The UK Armed Forces sometimes come off as a bit obsessed with procuring a Spot of Tea in the field; tea acts as a mild antiseptic on top of the sterilising effect of boiling the water.
An example of why this trope does not reflect reality is Harpur Hill quarry in Derbyshire, which, after a some decades of disuse, has filled with beautiful, clear, blue water which, due to caustic chemicals in the quarry stone has a pH of 11.3, which is roughly the level of bleach. Despite warning signs, people kept swimming in the "blue lagoon" (blue and clear due to its toxicity) and developing skin complaints and worse. Eventually, the local authority resorted to dyeing the entire place black to deter people.