Plan the water before the route feels fixed

Fill clean bottles with drinking water before leaving. Work out where you can refill, how long the longest gap is and what happens if the intended source is unavailable. A shop symbol on a map does not establish opening hours, and a tap at an isolated building is not automatically drinking water.

For a longer walk, confirm supply with accommodation, campsites or other known refill points in advance. Ask whether water is suitable for drinking and whether it is available at the time you will pass. Mark a shorter route or exit point if you cannot obtain enough water for the next stage.

The amount you need varies with duration, effort, weather and personal needs. Include water for any food preparation and for people or animals you are responsible for. Avoid selecting a single bottle size because it fits neatly in your pack, then treating that as your requirement.

Do not judge a stream by appearance

A stream can look clean while carrying contamination from livestock, wildlife, people or activity upstream. Running water is not a guarantee. You may also be unable to see the catchment beyond the next bend.

The West Highland Way published advice from Lomond Mountain Rescue in May 2026 recommending known drinking sources first and treatment before consuming stream water. It warns that filtration alone may not guarantee safety and recommends secondary treatment, especially where livestock or run-off may be involved.

Use that cautious approach when planning a walk elsewhere in the UK. Local supply advice and warning notices take priority over a general claim that upland water is safe. A dry period can also leave a mapped source unavailable; carrying a filter does not create water where none exists.

Match treatment to the hazard

Outdoor treatment products do different jobs. Read the manufacturer's information on what a device removes or inactivates, the water conditions it can handle and whether an additional step is needed. The words "filter" and "purifier" on a listing are not enough to establish performance.

  • A filter's ability to deal with bacteria, parasites and viruses depends on its design and specification. Do not assume that removing sediment makes water microbiologically safe.
  • Boiling can address microbial hazards, but does not remove chemical pollution. It also requires suitable equipment, fuel, time and safe handling.
  • Chemical treatments require the correct product, dose and contact time. Follow its instructions, including any limits for cold or cloudy water and suitability for the people drinking it.
  • UV devices depend on their operating conditions and power supply. Cloudy water can reduce effectiveness.

Mountaineering Scotland describes these treatment methods and their limitations. This guide does not give a universal dosing recipe because products and intended uses vary. Learn your chosen system before taking it on a route where you depend on it.

Know when to reject the source

Avoid water that appears polluted, has an oily film or algal growth, or is subject to a warning. Do not assume that a backpacking filter, tablets or boiling makes suspected chemical or algal contamination safe. Find a different drinking supply or change the route.

Do not use floodwater or water near a suspected spill as an improvised refill. The UK travel-health service NaTHNaC notes that chemical contamination is a separate water hazard. Its advice concerns travel more broadly, but that distinction matters when assessing outdoor treatment claims.

Collecting water should not expose you to a greater danger than the shortage. Stay away from steep banks, fast currents and unstable edges. If a source cannot be reached safely, it is not a usable source for your plan.

Keep treated water clean

Before the trip, practise the complete process using the manufacturer's instructions. Check how untreated water is collected, where treated water goes and which parts must stay clean. Label containers if there is any chance of confusing untreated and treated water.

Protect bottle mouths and caps from the untreated source, and keep dirty collection equipment separate. Wash hands where possible before preparing food or handling drinking surfaces. A treatment system can be undermined by pouring clean water into a dirty bottle.

Inspect filters, seals and other consumables before departure. Follow storage, cleaning and replacement instructions, including any requirements after freezing or damage. Carry the instructions offline if the process is unfamiliar, and consider a suitable backup for a longer route.

Change the walk before running dry

Check remaining water at planned stops. If an expected refill fails, reassess while you still have enough to reach a known supply or exit. Continuing towards a second unverified source makes the problem harder to solve.

If someone becomes unwell after drinking untreated water, seek medical advice and mention the exposure. Do not assume that treating the remaining bottle treats the illness. For household water storage, use the separate water storage guide; a walking filter should not become the entire household water plan.

Sources