Montgomery County rescue workers yesterday ended an eight-hour search for an 18-year-old Kensington man who is presumed to have drowned after a Saturday rafting accident in the rocky, treacherous waters of the Potomac River near Great Falls, officials said.

Rescuers found no trace of Joseph F. Welch of 10610 Wheatley St., who fell from the raft during an outing with a friend, according to Victor Esch, fire chief of Montgomery County's Cabin John station. The empty Army raft was discovered floating near the Carderock Recreation Area.

The victim's companion, 18-year-old Peter Metzger of the same address, was rescued Saturday by a U.S. Park Police helicopter and treated for exposure at Suburban Hospital in Bethesda. Police have said that the men were heading downstream in the raft when Metzger tried to grab a branch and lost his balance and both men fell in.

Metzger told authorities he last saw Welch going downstream. Welch has been described as an experienced swimmer and boater, but the eight-mile stretch of Potomac between Great Falls and the Brookmont dam is famous for its dangerous currents, strong undertows and rocks. Thirty people have drowned there since 1980, including five last year.