Drinking From Streams, Rivers and Other Sources Safely

No, stream water is not safe to drink as it comes, however clear and inviting it looks. NHS inform's summer health advice is blunt about this: germs including E. coli, Cryptosporidium and Leptospira can contaminate lochs, rivers and burns, and they are harmful if swallowed¹. UK Health Security Agency guidance on cryptosporidiosis says simply: "Do not drink untreated water."²

The catch is that you cannot rely on your senses. Scottish Borders Council warns that with private supplies, contaminated water may smell, taste and look normal, so you may not be able to tell whether water is safe³. A sparkling upland stream can still be carrying parasites from livestock upstream, and a river in spate after heavy rain can be carrying much more besides. The safe rule is to treat any water collected from a stream, river, loch, pond or spring before drinking it, and to know the signs that mean a source should be avoided altogether, such as a blue-green algal bloom.

This page covers which sources can be treated and which cannot, the main germs and what they do, how to recognise algae, how to keep children and animals safe at the water's edge, and what to do about water that has been standing in your own pipes and tanks.

Why clear water is not clean water

Public mains water in the UK is treated to make it safe to drink and has to meet stringent regulations, as the Drinking Water Quality Regulator for Scotland puts it⁶. A stream has had none of that treatment. Surface waters, meaning rivers, streams and lochs rather than underground sources, are considered to be at higher risk of contamination, especially with faecal bacteria, because any pathogens in them have had less time to die off compared with groundwater⁷. In plain terms: anything washed into a river yesterday may still be alive in it today.

What gets into that water matters. Cryptosporidium in manure can contaminate fields, rivers and crops², and Scottish Borders Council advises anyone collecting water to ensure it is not contaminated by discharges from a septic tank, slurry spreading, or nearby sources of pollution such as pesticide or oil storage³. Farm runoff, sewage discharges and dirty water from eroded land all feed nutrients and germs into watercourses. The same council lists the waterborne infections that private supply users face: Campylobacter, E. coli O157, Cryptosporidium and Giardia, all of which can cause diarrhoea and vomiting³.

The risk rises at certain times. Argyll and Bute Council notes there are occasions when there is an increased risk of harmful bacteria affecting any supply, most likely after heavy rainfall or snowmelt, or when the water is highly coloured⁸. During droughts, private supply quality can decline and supplies can become contaminated when water levels are low, which could make someone who drinks that water unwell⁹. So a stream that was safe to treat and drink last month may not be this month, and after floods or storms the usual treatment processes may not be adequate, particularly if the water is coloured or cloudy, in which case it may need to be filtered and boiled before use¹⁰.

The good news is that treatment works. NHS inform states that untreated water can be made safe to drink by boiling it or using chemical treatments¹, and the Drinking Water Inspectorate confirms that boiling is effective against parasites such as Cryptosporidium¹¹. For how to do this properly, see How to Make Water Safe to Drink in an Emergency and Boiling to Kill Germs: How to Do It Properly.

Cryptosporidium: the parasite untreated water can carry

Cryptosporidium is a parasite, a tiny organism, that causes an illness called cryptosporidiosis affecting people and some animals, particularly farm animals². It lives in the intestines and faeces of infected humans and animals, and it may contaminate lakes, streams and rivers, swimming pools, untreated or poorly treated water, food (especially raw milk and fresh produce), and objects such as farm gates, outdoor boots and clothing². That last part is worth pausing on: you can pick it up from a gate latch, not just from a drink.

You can get cryptosporidiosis directly from another person or animal by touching faeces, for example when changing a nappy; from swimming in or drinking contaminated water; and occasionally by eating contaminated food, including unwashed or unpeeled vegetables or salads, or drinking contaminated raw milk². The route that concerns this page is drinking from untreated, unprotected water supplies or swallowing contaminated recreational waters such as lakes, rivers and streams².

Because the parasite comes from faeces, anything that puts manure near water raises the risk. Cryptosporidium in manure can contaminate fields, rivers and crops², so streams running through grazing land, or downstream of farms, are exactly the kind of source to treat with the most care. Surface waters generally carry the highest faecal contamination risk because pathogens have had less time to die off than in groundwater⁷.

There is no way to tell by looking. Contaminated water may smell, taste and look normal³, and the only way to know you have cryptosporidium is for a doctor to ask for a sample of your faeces to be tested in a laboratory². If you have drunk untreated water and become ill, tell your GP about the water as well as the symptoms. For more on this parasite, see Cryptosporidium Outbreaks: What Households Should Do.

Symptoms of cryptosporidiosis and how long they last

Cryptosporidiosis symptoms include profuse watery diarrhoea, stomach pains, nausea or vomiting, low-grade fever and loss of appetite, which can lead to dehydration and weight loss². Argyll and Bute Council's sampling guidance describes the wider family of waterborne infections, including Campylobacter, E. coli O157, Cryptosporidium and Giardia, as producing acute gastrointestinal symptoms including sickness and diarrhoea¹². Not every upset stomach after a day out is one of these, but they are the ones to suspect when untreated water has been swallowed.

Symptoms usually last about 2 weeks, but can be longer, especially in people with weak immune systems². There is no specific treatment for cryptosporidiosis; most people with a healthy immune system will recover within one month². The illness is highly infectious, so you need to be very clean around your home for at least 48 hours after your symptoms stop, and you are infectious to other people while you are ill and have symptoms². You can also pass cryptosporidium in your faeces for several weeks even if you no longer have symptoms².

Hand hygiene is the main defence at home. UK Health Security Agency guidance says to wash your hands thoroughly with soap and warm water before preparing and eating food, after handling raw food, after going to the toilet or changing a baby's nappy, and after working with, feeding, grooming or playing with pets and other animals². Pay special attention to hygiene during farm visits, washing hands after any contact with animals, and eating only in designated areas².

If you work with vulnerable groups such as the young, the elderly or those in poor health, or if you handle food, you should tell your employer you have had cryptosporidiosis². If you are concerned about your health or your children, contact your GP; for further questions, contact your local health protection team².

Who is most at risk from contaminated water

Anyone can get cryptosporidiosis, but it is most common in children aged between 1 and 5 years². It can be a serious illness in people whose immune systems are not working properly². Argyll and Bute Council lists the vulnerable groups on private supplies as bottle-fed infants, the very young, the elderly and anyone whose immune system might be compromised⁸.

The government's water supply interruption guidance names similar groups who may be most at risk when supplies are affected: people who use more water due to a medical condition, babies and young children, older adults, and people with long term health conditions⁵. For these households the message is sharper: do not take chances with untreated sources at all, and make sure any stored or collected water has been properly treated. If your household includes a bottle-fed baby, see Babies and Formula Feeds When the Supply Is Off or Unsafe.

Some groups are more likely to be exposed in the first place. UK Health Security Agency guidance lists those at higher risk as people who handle infected livestock or their faeces, people exposed to human faeces through caring or sexual contact, people who drink from untreated, unprotected water supplies or swallow contaminated recreational waters, and people who travel to countries where these exposures are likely². For related help available to vulnerable households, see Extra Help in an Emergency: Older, Disabled and Medically Dependent Households.

Blue-green algae: how to recognise a bloom

Blue-green algae, also called cyanobacteria, are typically found in freshwater such as lakes, ponds and streams¹³. In still waters they can multiply to such an extent that they discolour the water, which then appears green, blue-green, greenish brown or dark brown, sometimes with a scum on the surface, most commonly at the water's edge or shoreline⁴. Blooms may colour the water or form a scum on the surface that may be smelly¹³.

Cumberland Council describes further signs: the water may look cloudy or discoloured, and foam may appear on the shoreline, which can be mistaken for sewage¹⁴. The algae may appear green, blue-green or greenish brown and often produce musty, earthy or grassy smells¹⁴. Blooms are more likely during long periods of warm, sunny weather and calm conditions with little or no wind¹⁴, and they commonly occur during prolonged hot weather, but sometimes occur at other times of the year⁴. They can be short-lived and transient, and may move around a body of water due to weather conditions⁴.

Waters affected by agricultural, domestic or industrial discharges can be prone to developing blue-green algae, but they can also occur naturally⁴. Reducing phosphate helps improve water quality, but algae can also use nitrogen and nutrients from dirty water and land erosion¹⁴. Look out for signs along the water's edge that may advise you not to use the water¹³.

Treat every algal scum as toxic

You cannot tell if a bloom is toxic just by looking at it, so it is safest to assume it is¹⁴. Argyll and Bute Council is firmer still: it is advisable to regard all algal scums as potentially toxic, because it is not possible to tell which algae produce toxins by appearance, and laboratory analysis is needed⁴. Public Health Wales agrees that it is not possible to tell by looking whether an algal bloom is harmful¹³.

The reason for the caution is real. Some blue-green algae can produce toxins, known as harmful algal blooms, which can harm wild animals, farm livestock and domestic pets, cause rashes after skin contact, and cause illness if swallowed¹⁴. Toxins in the blooms may cause skin rashes, nausea, vomiting, stomach pains, fever and headaches¹³, and exposure to high levels of toxic algae can cause more serious illness including liver damage¹³.

People and pets should avoid entering the water during a bloom¹⁴. Exposure routes are wider than swimming: being in or splashed by affected water; swallowing water or breathing in droplets when sailing, boating or fishing; water used for drinking; and touching pets, usually dogs, that have been in affected water¹³. Argyll and Bute Council's rule of thumb is short: "When in doubt, stay out."⁴

Keeping children, dogs and livestock away from affected water

Avoid all contact with affected waters and ensure that children are kept away⁴. Do not let your pets and other animals go into or drink the water or consume the scum⁴. Public Health Wales says the same: do not allow pets or animals into the water¹³. This is not an over-cautious rule. Dogs have died after going into the water at the shores of affected lochs, and affected animals can deteriorate very quickly after very little contact⁴. The toxins can cause deaths of animals through drinking contaminated water, swallowing quantities of scum or shoreline matter or crust, or licking contaminated coats⁴.

If you believe your pet may have been exposed to blue-green algae, contact your vet as soon as possible to get advice⁴. Farmers should ensure their animals do not have access to contaminated water, which may require fencing around suspect waters⁴. The Drinking Water Inspectorate makes the same point for private supply sources: install stock-proof fencing to protect the water source from grazing animals, including rabbits and deer as well as domestic livestock⁷.

The same care applies to flood water at the water's edge. The Met Office warns that flood water contains hidden dangers such as heavy debris, sharp objects, open manhole covers, sewage and chemicals, so keep children and pets away¹⁵. Public Health Wales says do not walk, drive, or allow children or pets to play in floodwaters¹⁶, and notes floodwater can be contaminated with sewage, chemicals and animal waste¹⁶. Don't let children or pets onto affected grass or paved areas until cleaned¹⁷, keep children and pets out of the affected area until the clean-up is complete¹⁸, and keep children out of recently flooded areas until they have thoroughly dried¹⁹. Wash children's hands regularly²⁰.

Stagnant water in tanks and pipes, and flushing before use

The risks are not only outdoors. Water that sits still in your own plumbing can become unsuitable to drink. The Drinking Water Quality Regulator for Scotland advises: do not drink water that has been standing in the pipes for long periods, for example overnight, or if no one has been in for several hours. In these circumstances, draw off a washing-up bowlful of water from the kitchen tap to clear the water which has been standing in the pipes²¹. This matters most where lead pipes are involved, but it is good practice in any home after a period away.

If your drinking water comes from a storage tank, it will be safe to use if the tank is properly designed, correctly installed and kept in good condition²². After floods or storms, check the supply from abstraction point to taps for damage, change filters more frequently because of suspended solids, clean storage, intermediate and loft tanks, and check collection points for dislodgement or blockage¹⁰.

Before using taps after a flood, check your drains and sewers are clear before flushing toilets and running taps²³. Any taps which have been submerged in contaminated floodwater should be cleaned using a bleach solution and run for 30 seconds prior to the water being used²⁴. Watch for the signs that the water running is fresh: it should run clear, at its normal temperature, and without the odd taste or smell that standing or contaminated water can leave. If discoloured drinking water persists, in Northern Ireland it should be reported to Northern Ireland Water²⁵. For more, see What to Do When Your Supply Comes Back On and How to Stay Safe from Legionella at Home.

Getting help: your GP, your vet and environmental health

If you think your water supply may be affecting your health, contact your GP³. In Argyll and Bute, the GP can provide containers for a stool sample, and the council water team can arrange to take a sample if appropriate¹². The only way to know you have cryptosporidium is for a doctor to ask for a sample of your faeces to be tested in a laboratory², so do not try to self-diagnose: describe the water you have drunk or the contact you have had, and let the test decide.

For pets, contact your vet as soon as possible if you believe your pet may have been exposed to blue-green algae⁴. For suspected algal blooms, take a photo and report it on the Bloomin' Algae website or app, and report it to your council's environmental health team⁴. Once a bloom is confirmed, weekly sampling will continue until 2 clear results are recorded¹⁴, so a report from you is what starts that protection for everyone else using the water.

If you are on a private supply, your council has a role too. Cornwall Council advises households to protect water quality by ensuring source protection measures remain effective, livestock cannot contaminate water sources, treatment systems are operating correctly, and filters and UV treatment systems are serviced and maintained²⁶. Check that pipework bringing the water from your source is in good condition, does not leak, and is protected from damage by weather, animals or farm machinery³. See Private Supplies: Wells, Boreholes and Springs for the full picture.

Collecting rainwater, and what not to drink

Rainwater collected at home is not drinking water. The government's advice on supply interruptions is clear: do not drink water from a rain butt or tank as it could contain bacteria or disease. It can be used for flushing toilets or watering plants⁵. That is a genuinely useful role in an interruption, and it is covered in How to Flush the Toilet During an Outage, but it is not a substitute for a treated drinking supply. For drinking, see How Much Drinking Water to Store per Person and Storing Tap Water Safely at Home.

If you harvest rainwater as part of a private supply, the Drinking Water Inspectorate advises that approved materials (WRAS or permitted under regulation 5) should be used to prevent contamination of rainwater during runoff and collection⁷. Protect the source as you would any other: ensure the water being collected is not contaminated by discharges from a septic tank, slurry spreading, or nearby sources of pollution such as pesticide or oil storage³. For the wider picture on rainwater and living off mains, see Self-Reliance at Home: Rainwater, Off-Grid Power and Living Without Mains.

For treating water you have collected, chemical treatments are an alternative to boiling¹. LifeSystems states that its chlorine dioxide drops kill bacteria, viruses and cysts in water including Giardia and Cryptosporidium²⁷. See Purification Tablets: Chlorine, Chlorine Dioxide and Silver and Emergency Filters: Gravity, Pump, Bottle and Straw Types for the options, and What Each Filter Removes for what different filter types actually take out.