The risk for mosquito activity is extremely high. Wear long-sleeved shirts and pants when spending time outdoors to prevent bites.
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Invasive species have a negative impact on natural resources nationwide, including in Yosemite National Park. Non-native animal species, like the New Zealand mud snail, concern park scientists because this species can completely cover a river streambed, thereby altering the ecosystem.
American black bears found in Yosemite National Park have long been of intense interest to park visitors and managers. Seeing one of the approximately 300 to 500 black bears in Yosemite can evoke excitement, awe, and fear. If visitors spot a bear while in the park, it is a black bear-not a brown or grizzly bear.
While true altitude sickness is normally only the concern of mountaineers heading for high summits, you can experience mild symptoms from the elevation in Yosemite—headaches, shortness-of-breath, and the like—as low as 5,000 feet. Staying a step ahead of altitude sickness means taking things slow.
Stay away from river and creek banks during high water conditions and avoid rock hopping. Stream polished rocks along the water's edge may be slippery when wet or dry. If you choose to cross a stream without a bridge, avoid deep and/or swift water.
Yosemite National Park offers a range of swimming opportunities—from rivers and lakes to our family-friendly pools, which are the only swimming places in Yosemite with lifeguards. River and lake swimming is at-your-own-risk. Life jackets are strongly recommended—especially for children.
The deer mouse (Peromyscus maniculatus) is the primary reservoir for Sin Nombre Virus, the strain of hantavirus responsible for the human cases in Yosemite National Park, and most human cases in the United States.