Assistant park manager Stacie Hall says the family of 29-year-old Jeremy Walker, of Somerset, hasn't heard from him since New Year's Eve.
Crews began searching the park, about 50 miles southeast of Pittsburgh, after the man's car was found parked there Wednesday.
The park covers more than 20,000 acres and includes whitewater rapids on the river.
Crews are focusing their search there because they say a dog traced Walker's scent from his car to a bridge on the river.
Crews say they've painstakingly searched about half of the area within three-quarters of a mile of his car to no avail.
Walker is known for previously visiting the park.