For any emergency in Yosemite National Park, dial 911 from most phones hotel and retail phones may require 9+911. Yosemite NPS runs its own dedicated emergency dispatch. If you dial from a cell phone, first mention that you are in Yosemite as many cell phones route to a call center in your number's area code
fees/permits
Park entrance fees are $20 for private vehicles and $10 for individuals on foot, bike and motorcycle. All entrance fees are valid for seven days. The Yosemite Annual Pass is available for $40, allowing park entry for one year. Alternatively, The National Parks and Federal Recreational Lands Annual Pass can be purchased for $80, allowing free entry to all park and recreation lands controlled by the US Department of the Interior for one year.
wildlife
Over the years the park's bears have become accustomed to scavenging trash and food left out by humans, and will even break into cars and tents to get it. While not the larger grizzly bears that once roamed California, black bears are strong enough to tear a door off of a car with ease. Luckily they usually prefer to avoid humans, so they'll most likely do their work on vehicles left at trailheads or in parking lots. Prevention is remarkably simple: never leave food or scented items deodorant, air fresheners in your car or bring them into your tent. Heed this advice! Leaving even just a tube of toothpaste or empty food wrappers in a car may result in thousands of dollars of damage to your vehicle should a bear choose to investigate the smell! Bear-resistant storage units are provided at park campgrounds and overnight parking areas: use them.
To avoid bear encounters while hiking, make noise so that the animal knows you are coming. This approach will also help to avoid encounters with mountain lions, which also inhabit the park. Other animals, such as the herds of deer which can be found in the park's meadows, can be equally dangerous; a young boy was killed by a deer in Yosemite Valley several years ago. Give all animals their space, and never feed any park wildlife.
other concerns
Currently, the greatest danger in the park comes from the thousands of park visitors. Petty thieves and traffic accidents are two issues to be aware of. Follow park speed limits, lock your vehicle, and be aware of your belongings, especially in Yosemite Valley. Violent crime is extremely rare in Yosemite, but given the numbers of people that visit you should expect that a few unsavory characters will be visiting too.
The National Park Service provides the primary law enforcement and fire protection in the park. NPS is supplemented by DNC Security, who handle a number of calls for service on DNC land assignments. DNC Fire is paged out along with NPS Fire, and handles a large number of calls in Yosemite Valley.
Lost and Found
There are two major Lost and Found operations in Yosemite. One is run by the National Park Service. It can be reached at 209-379-1001. The other is run by DNC, and can be reached at: 209-372-4357. They coordinate as best as possible, considering they are a half-hour away from each other. They process thousands of items each year, and surprising, get a number of items back to the rightful owner. Because of the thousands of items lost or found, generally, you will not get a return call unless your item has been found and turned in correctly.
Items that are found that cannot be returned to the owner are generally turned over to recognized charities. So, if you don't get your item back, and if it is turned in by the finder, at least you can rest easy that it will eventually go to a good cause.
on the trails
Hikers should follow all posted signs - if a trail is closed due to ice, landslide, or some other reason do not ignore the closure as doing so endangers both the hiker and any area that must be traversed to go around the closure. On the Half Dome trail hikers should ALWAYS remain inside the cables for both their ascent and descent - people have died on this section of trail as recently as Summer 2009.
The park's waterfalls pose another potential hazard. Do not attempt to get close to the waterfalls, especially in the spring. This includes swimming above the waterfalls at a distance of less than 1 mile about 1600 meters The force of the water will easily sweep a person off their feet and over the falls. Being swept over any of Yosemite's waterfalls is invariably fatal.
light
Yosemite Lodge and possibly other accommodation areas has no outside lighting. This is to reduce light pollution and allow the stars to shine down. If moving about the area at night even to go to the reception office or restaurant ensure that you have a flashlight torch, as walking in the dark can be quite hazardous.
weather
Other natural dangers in the park come from the weather. Hypothermia can be a concern at higher elevations where temperatures can drop below freezing throughout the year. Dress in layers, and be prepared for storms and rapid changes in temperature. When storms are approaching avoid open areas such as the summits of the park's many granite domes; lightning strikes these areas regularly. If a storm does approach, get off of high, open ground. When hiking wear sturdy footwear and drink plenty of water - if you are thirsty that is an early sign of dehydration. Be aware that the sun can be intense at higher elevations and when reflected off of snow, so sunscreen is important. In the winter, take the weather term "Winter Storm Warning" very seriously as it means a significant storm is definitely coming.