The risks of wilderness travel or danger from accidents, wildlife, and natural phenomena must be accepted as part of the wilderness experience. Common sense and some knowledge of safe travel and camping techniques are required to protect yourself and others from harm. Be alert to the following situations:

Weather

Sudden changes in weather catch many unaware. Drenching thunderstorms can form in a matter of hours, and snow can fall at any time during the year. Be prepared for all weather conditions. Afternoon thunderstorms are a frequent and spectacular occurrence in the Yosemite wilderness. These summer storms often bring intense rain, hail, and lightning strikes, particularly in mid to late afternoon. Plan to be over passes and away from high open areas by noon. During a storm, stay away from peaks (particularly Half Dome), ridges, caves, water, and open areas. Seek shelter in low forested areas, but avoid tall, solitary trees. By setting up camp in a safe location before lightning begins, you can enjoy the power and spectacle of a mountain thunderstorm without apprehension.

High water

During early spring and summer, run-off from melting snow can cause high water levels and swift currents in rivers and streams. Please remember that any unbridged stream crossing may be hazardous. Cross in a wide shallow spot that is not above rapids or falls. Unbuckle waist straps, use a long stick for stability and face upstream while crossing. Don’t tie yourself in to "safety ropes" - they can drown you. Water will be extremely cold.  Caution should be used to prevent conditions which may lead to hypothermia.

Tick borne diseases

There are two known diseases carried by ticks in this area: Lyme disease and Relapsing fever (borelliosis). Not all ticks carry these diseases. If you are bitten by a tick, and later experience flu-like symptoms, contact your doctor and mention you had a tick bite. If you are diagnosed as having Lyme disease or Relapsing Fever, and you believe you got it in Yosemite, have your doctor contact the park sanitarian at (209) 372-0206.

Giardia Lamblia

Giardiasis is an intestinal disease caused by Giardia lamblia, a water-borne protozoan. Giardia is carried by humans and some domestic and wild animals that may contaminate lakes and streams. All water or melted snow must be treated by boiling for at least 5 minutes, using an iodine-based purifier, or using a Giardia-rated water filter. Associated symptoms include chronic diarrhea, abdominal cramps, bloating, fatigue, and loss of weight. Treatment by a physician is necessary to kill the organisms. Life Cycle of Giardia (big) Life Cycle of Giardia (small)