Water Safety

Rivers and Lakes

Water is the number one cause of fatalities in the National Park System. Use extreme caution near water. Swift, cold glacial waters of the San Joaquin River, moss-covered rocks, and slippery logs all present dangers. Children, photographers, swimmers, and fishermen have fallen victim to these rapid, frigid streams and deep glacial waters. Avoid wading in or fording swift streams. Never walk, play, or climb on slippery rocks and logs, especially around Rainbow Falls. Below are some helpful hints to remember when looking for an area to swim:

  • Always supervise children closely.
  • Choose swimming areas carefully and swim only during low water conditions.
  • Avoid areas of whitewater, where streams flow over rocky obstructions.
  • Never swim or wade upstream from the brink of a waterfall, even if the water appears shallow and calm. Each year unsuspecting visitors are swept over waterfalls to their deaths when swimming in these areas.
  • Never swim or wade underneath a waterfall. Dangerous currents can pull even the best swimmer underwater and cause them to drown.


Hypothermia, the “progressive physical collapse and reduced mental capacity resulting from the chilling of the inner core of the human body,” can occur even at temperatures above freezing. Temperatures can drop rapidly. Exposure to frigid bodies of water and sudden mountain storms can turn a pleasant day into a bitterly cold and life-threatening experience. People in poor physical condition or who are exhausted are particularly at risk.

Preventing Hypothermia

  • Avoid hypothermia by using water-resistant clothing before you become wet.
  • Wear clothing that wicks moisture away.
  • Minimize wind exposure and if your clothes become wet, replace them.
  • Avoid sweating by dressing in layers, rather than in a single bulky garment.
  • Pack a sweater, warm hat, and raingear for any hike.

The Warning Signs

  • Uncontrolled shivering
  • Slow or slurred speech
  • Memory lapses and incoherence
  • Lack of coordination such as immobile or fumbling hands, stumbling, a lurching gait, drowsiness, and exhaustion.

Immediate Treatment

  • Seek shelter from weather and get the victim into dry clothes.
  • Give warm non-alcoholic drinks.
  • Build a fire and keep victim awake.
  • Strip victim and yourself, and get into sleeping bag/dry clothes making skin-to-skin contact.
  • If victim is semi-conscious or worse, get professional help immediately.


Sudden immersion in cold water (below 80°F / 27°C) may trigger the “mammalian diving reflex.” This reflex restricts blood from outlying areas of the body and routes it to vital organs like the heart, lungs, and brain. The San Joaquin River averages temperatures from 30°–60° F (-1°–16°C), so the potential for a life threatening situation exists.

Drinking Water Cautions

Giardiasis is caused by a parasite (Giardia lamblia) found in lakes and streams. Persistent, severe diarrhea, abdominal cramps, and nausea are symptoms of this disease. The time between being infected and developing symptoms is 7–14 days. The acute phase lasts 2–4 weeks. If you experience any symptoms, contact a physician.

When hiking, carry water from one of the park’s treated water systems. If you plan to camp in the backcountry, follow recommendations received with your permit. Bring water to a boil or use an approved filter.

Harmful Algal Blooms

Small amounts of toxic algae have been found above and below Rainbow Falls during times of low water, and toxic algae could exist in other sites in the monument. Toxins are concentrated within the algal mats themselves and released episodically into the water when the algae dies or is disturbed.

What does this mean for me?

  • For your safety, do not enter or drink in the area of Rainbow Falls if you see any algae in the water.

  • Filtering and/or boiling the water is not effective against this type of algae.

  • Prevent pets from drinking the water and eating or touching algae in the water and dried on the shore. In particular, prevent dogs from eating dried algal mats on shore.

  • Please report any large algal blooms and/or algae that is particularly bright, bubbly, strange-looking, or appears like a haze in the water.

  • Do not disturb algal mats in any way. Wading or swimming can cause toxins to be released into the water.

  • If you suspect a site has toxic algae, do not enter the water and do not drink water from the area. While some sites are signed based on testing results, it’s likely that algae exists in other parts of Devils Postpile. Don’t rely on signage alone.

Can I still swim in the river? Can I still filter or treat the water for drinking?

Water that is clear with no visible algae in the area presents a low risk. Even in areas with no visible algae, watch for isolated clumps of algae floating by.

If you think algae may be in the water:

  • Do not enter the water.

  • Do not drink the water, even if treated.

  • Do not let pets into the water, allow them to drink the water, or eat algae on the shore.

What are the signs and symptoms of exposure to toxins from algae?

According to the California Water Quality Monitoring Council, the following signs and symptoms may occur within 48 hours of exposure to a waterbody with a suspected or confirmed algal bloom:

  • sore throat or congestion;

  • coughing, wheezing, or difficulty breathing;

  • red, or itchy skin, or a rash;

  • skin blisters or hives;

  • earache or irritated eyes;

  • diarrhea or vomiting;

  • agitation;

  • headache; and/or,

  • abdominal pain.

If people show symptoms of cyanotoxin and/or cyanobacteria exposure after contact with water, or with scums or mats of algae, they should receive immediate medical attention. Additional resources are available from the Centers for Disease Control and Prevention (CDC), and by contacting the California Poison Control Center (1-800-222-1222). See the HAB-related Illness Tracking webpage for information on previously reported human illnesses related to HABs in California.

Is the National Park Service monitoring for harmful algal blooms?

The park continues monitoring for toxic algae and testing for toxins throughout the park. You can learn more about harmful algal blooms at https://mywaterquality.ca.gov/habs/.

Last updated: October 25, 2021

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