YOSEMITE NATIONAL PARK (CNN) -- A 10-year-old boy drowned and a 6-year-old was missing after a strong current swept the children down a boulder-strewn river in California's Yosemite National Park, park officials said.
The two boys and other family members waded into the Merced River near the Vernal Fall Footbridge after hiking on Mist Trail a mile from the Happy Isles Trailhead on Wednesday afternoon, a National Park Service statement said.
"Approximately 150 yards downriver, the 10-year-old boy was pulled from the river and CPR was initiated by a park visitor," the statement said. Park rangers "continued resuscitation efforts, which were unsuccessful, and the boy was pronounced dead."
A search for the 6-year-old began immediately and continued into Thursday afternoon, it said.
The children, whose names were not released, are from Southern California, the statement said. The footbridge is on a trail on the way to Vernal Fall, according to the park.
Park spokesman Scott Gediman told CNN Sacramento affiliate KCRA-TV the river is shallow near shore, but four to five feet deep at some spots in the middle. "The current is very deceiving."
"It's a real rough terrain and swift-flowing water flowing around rocks," said Gediman. "It's really difficult search conditions, but we're making every effort we can to locate this young boy.
In July 2011, a young man lost his footing, slipping close to the edge of the Vernal Fall waterfall. A female companion frantically grabbed for him but stumbled. Another hiker followed and the three died after they were swept over the powerful 317-foot falls.
One body was found in August and two others were recovered in November and December 2011. Yosemite offers visitors spectacular views of mountains, falls and rivers, but park officials warn visitors to be careful and adhere to caution signs.
Rangers say some visitors partake in dangerous practices such as hiking treacherous trails in flip-flops, climbing over safety rails to take better pictures or swimming perilously close to waterfalls. Witnesses told park authorities that the three hikers who were swept over the waterfall in July 2011 had climbed over a safety rail.
"We don't station a ranger in every possible dangerous place that's out there," park ranger Kari Cobb said last year. "People have to come here and realize that Yosemite is nature, and it is a very wild place."