In an extreme instance in April, two young men from Las Vegas were killed in Zion National Park in Utah while trying to float a hand-built log raft down the Virgin River. A park investigation found that the men “did not have whitewater rafting experience, and had limited camping experience, little food and no overnight gear.”

“They told their father that they intended to record their entire trip on video camera as an entry into the ‘Man vs. Wild’competition” on television, investigators wrote.

Far more common but no less perilous, park workers say, are visitors who arrive with cellphones or GPS devices and little else — sometimes not even water — and find themselves in trouble. Such visitors often acknowledge that they have pushed themselves too far because they believe that in a bind, the technology can save them.

It does not always work out that way. “We have seen people who have solely relied on GPS technology but were not using common sense or maps and compasses, and it leads them astray,” said Kyle Patterson, a spokesman for Rocky Mountain National Park, just outside Denver.

Like a lot of other national parks, Rocky Mountain does not allow cellphone towers, so service that visitors may take for granted is spotty at best. “Sometimes when they call 911, it goes to a communications center in Nebraska or Wyoming,” Mr. Patterson said. “And that can take a long time to sort out.”

One of the most frustrating new technologies for the parks to deal with, rangers say, are the personal satellite messaging devices that can send out an emergency signal but are not capable of two-way communication. (Globalstar Inc., the manufacturer of SPOT brand devices, says new models allow owners to send a message with the help request.)

Image In a video on YouTube, Cathy Hayes showed her injuries and recounted being charged by a buffalo at Yellowstone National Park while filming a companion.

In some cases, said Keith Lober, the ranger in charge of search and rescue at Yosemite National Park in California, the calls “come from people who don’t need the 911 service, but they take the SPOT and at the first sign of trouble, they hit the panic button.”