Officials are searching for a hiker who went missing in California's Yosemite National Park last weekend.

Hayden Klemenok was backpacking with a group of friends at Upper Chilnualna Falls when he entered Chilnualna Creek near the trail junction Sunday afternoon, according to a Wednesday Facebook post from the park.

Klemenok has not been seen since, the park said.

He was wearing a tan-colored brimmed hat, white sunglasses, a red T-shirt, blue swimming trunks and white Adidas shoes on the day he went missing, park officials said.

Officials urged visitors to contact Yosemite Park dispatch if they have seen Klemenok or were hiking near the area he was last seen on Sunday.

"Simply knowing where you went and when you were there may help us focus the search, whether you saw anyone or not," the park's post said.

A family friend created a GoFundMe for Klemenok's family "to help ease the pain of the costs of all the expenses and struggles they are currently facing." The family is taking time off to help search for Klemenok, the fundraiser page said.

The fundraiser described Klemenok as "a truly extraordinary young man" and a recent graduate of San Diego State University.

A separate alert issued by Yosemite officials warned that water-related accidents are the second-leading cause of park deaths, and that "rivers are surprisingly swift, cold and dangerous."

"Each year, 15 to 20 visitor rescues are directly associated with unprepared victims finding themselves in the water," the park said.