Water Safety

Rivers and Lakes

Water is the number one cause of fatalities in the National Park Service. 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° F-60° F, so the potential for a life threatening situation greatly increases.

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.

Last updated: March 31, 2012

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PO Box 3999
Mammoth Lakes, CA 93546


(760) 934-2289

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