Finding water in an emergency

01 — Evaluating natural sources

Flowing water is generally safer than still water. Appearance still tells you nothing about biological safety.

The EPA's emergency disinfection guidance states that when natural water sources must be used, flowing water is generally preferable to still, stagnant water. Moving water is less likely to concentrate pathogens and organic matter than a pond or puddle that has sat undisturbed. But preferable does not mean safe without treatment: all natural water sources, regardless of appearance, must be treated before drinking.

The EPA also specifies indicators to avoid regardless of source: water with floating material, a dark color, or a questionable odor should not be used even with treatment. These signs suggest contamination levels or types that standard field treatment cannot reliably address. When multiple sources are available, choose the clearest, fastest-moving water from the highest point accessible, away from human activity, animal grazing, and any visible discharge points.

02 — Pre-filtering natural sources

Always filter visibly contaminated water before disinfecting it

The EPA and CDC both specify pre-filtering as the first step when treating any natural water source that is cloudy or contains visible sediment. This is not optional. Particles in the water physically shield pathogens from heat and chemical disinfectants, reducing treatment effectiveness. Pre-filtering removes the physical protection before treatment is applied.

The method does not require specialized equipment. Pour the water through a clean cloth, a paper towel, or a coffee filter into a clean container. Allow heavily silted water to sit undisturbed for several hours first, then carefully decant the clearer water from the top before filtering. The goal is to reduce turbidity to the point where the water looks reasonably clear before boiling or chemical treatment begins.

After filtering, treat the water by boiling for one minute (three minutes above 6,500 feet elevation) or by chemical disinfection. Boiling is the more reliable choice when the source may contain Cryptosporidium, which is resistant to chlorine and iodine at field concentrations. Pre-filtering alone does not make water safe to drink.

• If the water is visibly silty or cloudy, let it settle in a container for several hours. Pour off the clearer water from the top.
• Filter the water through a clean cloth, paper towel, or coffee filter into a second clean container.
• Boil the filtered water for one minute at a full rolling boil (three minutes above 6,500 feet). Or treat with bleach per EPA dosing guidance.
• Let the treated water cool in a covered container before drinking or storing.

03 — Snow as a water source

Melt snow before you use it. Do not eat it directly.

Snow is a usable emergency water source with two important constraints. The National Weather Service guidance and multiple emergency management sources are direct on the first: do not eat snow to meet your water needs. Eating snow lowers your core body temperature, which increases risk of hypothermia. In winter emergencies, where hypothermia is already a concern, this trade is a poor one. Melt snow first, then drink the water.

The second constraint is volume. Snow is mostly air. Depending on snow density, it typically takes about 10 to 20 parts of snow to produce one part of liquid water. A full pot of packed snow may yield only a few cups of water. This means using snow as a primary water source requires sustained effort and fuel, and should be factored into any conservation plan.

Freshly fallen snow from a clean area away from roads, buildings, and industrial activity is generally safe after melting and can be used without treatment in most cases. Snow that has sat on the ground, been exposed to wind-blown debris, or collected near contamination sources should be treated after melting, the same as any natural water source. If the snow is discolored, avoid it.

04 — Sources to avoid

Flood water and chemically contaminated sources cannot be made safe by standard field treatment

FEMA's flood safety guidance warns that flood waters carry waste and pollute drinking water. The EPA is equally clear: boiling and chemical disinfection address biological threats but will not destroy heavy metals, salts, pesticides, or most other chemicals. When flood water combines biological and chemical contamination, no field treatment method makes it safe to drink.

The same logic applies to water from areas with known industrial or agricultural contamination. A visually clear stream running through farmland may carry nitrates, pesticides, or herbicides at concentrations that treatment cannot address. Without laboratory testing, there is no reliable field method to assess chemical contamination. In these situations, sealed bottled water or water from a confirmed uncontaminated source is the only safe option.

A flooded private well is a related concern. Floodwater can submerge wellheads and contaminate the water supply. The EPA recommends having a flooded well disinfected and tested by a professional before use. Contact your state or local health department for specific guidance.