The Sierra Nevada mountain range provides breathtaking views and is among the best in the world for summer climate. Not surprisingly then, the 505-mile portion of the Pacific Crest Trail (PCT) from Walker Pass to Highway 80 north of Lake Tahoe is a very popular and well-traveled stretch of trail. The area has thousands of lakes and streams which provide life supporting water while also enhancing the beauty of the alpine experience. But is this water safe to drink untreated?

The Good News

The Sierra Nevada watershed provides California with nearly 50% of its fresh water. This means that many agencies are interested in ensuring that the water is clean and pure. Most of the precipitation in the Sierra falls in winter as snow which serves as a frozen reservoir for much of the year. Most Sierra water is clear and pure with only a few microorganisms capable of causing diseases in humans.

Viruses: Viruses which cause human disease are called pathogenic viruses. Most pathogenic viruses can not survive in the environment. Even those rare pathogenic viruses that withstand environmental stress generally do not do well in the open, exposed, harsh environment of the Sierra. So, although certain viruses such as Norwalk Virus (Cruise Ship Virus), polio virus, Hepatitis A, and Rota virus have the ability to sustain viability on some external surfaces, survival in the Sierra landscape would be very difficult. Furthermore, no documented cases of viral acquired illnesses have been traced from drinking water from Sierra streams or lakes. Water filtration or purification to kill viruses in most backcountry acres of the Sierra is not necessary. Viruses which do not cause human disease (non-pathogenic) differ from pathogenic ones in that most are environmentally hardy. Many species of these exist, including viruses (or phages) that feed on bacteria, protozoa, algae, and plankton, but these are of no concern to the PCT hiker.

Protozoa: In the 1980s backpacker literature was filled with the warnings about Giardia in the Sierra. Giardia was considered the major harmful water microbial contaminant in wilderness areas, but the significance of Giardia has been exaggerated (1). On average, one would have to drink 250 gallons of Sierra water to become ill (2). In one study of Sierra Nevada backpackers who developed diarrhea, no one had proven Giardia. Giardia is a zoonotic type of organism which also infects many non-human mammals. We have analyzed multiple specimens of pack animal manure along the John Muir Trail and have not found significant amounts of Giardia. Furthermore, other studies examining horse and pack animal manure of animals entering backcountry trails has failed to yield significant amounts of Giardia. If one wants to be entirely safe, one can purify water, even though perhaps less than 1% of streams in the Sierra may have Giardia significant enough to cause infection in humans.

Sierra water also contains many species of protozoa not capable of causing disease in humans. These microorganisms are an important part of the ecosystem and serve as a food source for the many types of plankton.

Sunlight: The UV rays from sunlight are powerful killers of microorganisms (3). For this reason, the first twelve inches of surface lake water have the fewest microorganisms. In nearly 300 samples of water from Sierra wilderness areas, our research group consistently found fewer total bacteria in lake surface water when compared to streams. In addition to sunlight, other factors may also reduce bacteria including settling effects, or ingestion of bacteria by zooplankton or other small organisms.

Potential Hazards

Bacteria: Multiple species of bacteria can cause water-borne disease in humans. Water in lakes and streams below areas where cattle graze pose the greatest risk to PCT hikers. Another potential risk is the presence of pack animals that defecate directly into the water. Also manure may be swept into streams and rivers by both summer storms and annual snowmelt. Rarely, bacteria may originate from natural, wild animal zoonotic reservoirs. Harmful bacteria include certain strains of E-coli, Salmonella, Campylobacter, Yersinia, and Aeromonas.

The most common species associated with human or animal fecal contamination is E-coli. It is generally accepted that E-coli and other coliform bacteria can survive in aquatic environments for several weeks to months depending on the nutriment availability, pH, and water temperature. A recent study of the beaches of Lake Michigan suggests that E-coli may sustain itself indefinitely in appropriate environmental situations (4). Indeed, we have found significant concentrations of E-coli below cattle grazed meadows in the Golden Trout Wilderness nine months after the last cattle grazing activity (5). Range cattle are noted to carry the potentially lethal E-coli strain O157:H7 at a rate of 1%, potentially placing those who drink untreated water below established cow pastures at risk for a very serious pathogenic disease. In addition, many non-O157 E-coli are capable of inducing serious disease in humans. We have found low levels of E-coli in one-fourth of water samples throughout the Sierra, primarily below cattle grazing tracts and heavily utilized campsites. In addition, we have found the organism Yersenia in many high alpine lakes of the Sierra, most likely as a result of fecal contamination from migrating flocks of birds (6).

Non-harmful Bacteria: Most aquatic bacteria are naturally occurring and part of the normal ecosystem of lakes and streams. On average 10,000 to 20,000 bacteria are found in each liter of water in the Sierra. These normal aquatic bacteria are not harmful to humans and actually serve an important niche in the ecological chain. They may fix nitrogen to be utilized in higher forms of life and are also ingested by Sierra zooplankton. Indeed, if bacteria were absent the normal food chain from frogs to fish as well as the ecological balance would be in jeopardy. A previous study of wilderness water has suggested a correlation between total bacterial counts and usage by backpackers.

Should one treat water to kill bacteria? This is a personal decision and depends on the risk the individual wishes to take. We believe the risk is low. I treat my water in only two situations: 1) below cow and sheep pastures and 2) in slow flowing, warm streams immediately below heavily utilized campsites.

Threats to Sierra Water

Water Clarity: Visible algae and algae blooms have increased in remote wilderness lakes and streams over the past 30 years. Abundance of algae in the water and at the bottom of the lake is an indicator of eutrophication (excess nutrients). In the Sierra, lakes are naturally nutrient limited, resulting in clear, deep blue, oligotrophic lakes. In such lakes, there is little algal biomass and the natural ecosystem is based on this condition. However, nutrient loading, primarily with phosphorous runoff, can lead to excessive overgrowth of algae. Although algae produce oxygen by photosynthesis, excessive growth reduces light reaching the bottom layers of the lake, causing reduced photosynthesis in the deeper layers. Decomposition uses all the oxygen in the water and the lakes become partially or entirely anoxic. This condition of no oxygen radically alters the ecosystem, killing fish and other animals that all rely on oxygen.

Multiple possibilities may explain the increases in Sierra algae, including 1) deposition of soaps and sunscreens from backpackers, 2) increases in atmospheric nitrate compounds (Nox) from central valley air pollution, 3) manure from cattle and pack animals, and 4) climatic changes. Use of soaps, sunscreens and other substances containing chemicals by backpackers may induce excess algae growth. PCT hikers can help by not washing off sunscreens into lakes and streams. Similarly many soaps and detergents contain substances harmful to the environment, even though they may be biodegradable. For example, phosphate is at the end of certain biodegraded chains, and excess phosphate is harmful to the Sierra environment by inducing excess algae growth.

Conclusion

Based on our research, it's apparent that many Sierra Nevada mountain backcountry lakes and streams have clean water and good clarity. Still there are risks, with grazing cattle posing the greatest threat to water safety. In some areas, coliform pollution from pack animals or backpackers may create a risk, as well. Additionally, increases in algae in lakes and streams pose a threat to water clarity and a healthy watershed.

The decision to treat drinking and other water while in the backcountry is a personal one that is best based on a knowledge of risk factors and what's necessary for you to have peace of mind. Please consider the research presented in this article with that in mind.

Robert W. Derlet, MD, is Professor and Former Chief of Emergency Medicine at UC Davis Medical Center in Sacramento, California. His Sierra water quality research is sponsored by a grant from the Wilderness Medical Society.

REFERENCES

1. Rockwell R. Wilderness water purity, especially in the High Sierra. The American Alpine News 11:(238) Summer 2002.

2. Zell SC, Sorenson SK. Cyst acquisition rate for Giardia lamblia in backcountry travelers to Desolation Wilderness, Lake Tahoe. J Wild Med 4:147-154, 1993.

3. Bourrouet A, Garcia J, Mujeriego R, Penuelas G. Faecal bacteria and bacteriophages inactivation in a full-scale UV disinfection system used for wastewater reclamation. Water Sci Technol 43(10):187-94, 2001.

4. Whitman RL, Nevers MB. Foreshore sand as a source of Escherichia coli in nearshore water of a Lake Michigan Beach. Appl Environ Microbiol 69(9):5555-5562, 2003.

5. Derlet RW, Carlson JR. Incidence of fecal coliforms in fresh water from California wilderness areas. Proceedings of the American Society for Microbiology, Washington D.C., May 18-22, 2003.

6. Derlet RW, Carlson JR. An analysis of wilderness water in Kings Canyon, Sequoia, and Yosemite National Parks for coliform and pathologic bacteria. Wild & Environ Med. In press.