San Bernardino County Sheriff's deputies say 69-year-old Barbara Thomas was hiking with her husband Friday when the two got separated.
They were about 20 miles north of Interstate 40, east of Kelbaker Road.
Thomas was last seen wearing a black bikini, a red baseball cap, and tan hiking boots.
She doesn't have any supplies or a cell phone. Temperatures in the area climbed above 100 degrees Saturday.
Anyone with information is urged to call the San Bernardino County Sheriff's Department's Colorado River Station at 760-326-9200 or Sheriff's Dispatch at 760-956-5001.