Hayden Klemenok was last seen backpacking with a group at Upper Chilnualna Fall on Sunday afternoon.
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Park officials say he entered Chilnualna Creek near the trail junction at about 2 p.m. but hasn't been seen since.
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He was last seen wearing a red t-shirt, blue swimming trunks and white sneakers with a tan hat and sunglasses.
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If you see him, contact Yosemite National Park dispatch.
According to Klemenok's LinkedIn, he is from the Bay Area and played three years of baseball at Santa Rosa Junior College.
Stay with ABC7 News for the latest details on this developing story.