A 16-year-old Rockville youth was believed drowned in the Potomac River near Great Falls yesterday, the fifth river drowning victim in 12 days.

The youth, whose identity was withheld by police, reportedly had been playing on the rocky banks of the Maryland shore when he fell into the high-water rapids below in an area known as Catfish Hole about 1:30 p.m. The incident prompted authorities to issue fresh warnings about the river's hazards.

The Potomac, its currents strengthened by recent rains is "definitely up," said National Park Service Ranger Michael Brown. "In fact, we almost lost a ranger today in the search for that boy."

It was in the same area, near Catfish Hole below Great Falls, that a man and a woman were swept away by the current last Sunday afternoon.

"First of all, there's no swimming allowed in the Potomac," Ranger Brown said last night. "And then, people who do get into the water don't realize there's a terrific current until they're in it. It's virtually impossible to swim out of it."

The body of the woman who drowned Sunday was found yesterday floating near Carderock Spring down-stream less than an hour after the Rockville youth disappeared. She was identified by a funeral home as Brenda Barley, 20, of Bricktown, N.J. The man, whose body was recovered near Old Angler's Inn on Wednesday, was identified by police as Robert Prentice, 33, of 9320 19th Ave., Adelphi.

During yesterday's search for the missing 16-year-old, Park Ranger David Jones' canoe overturned and he was rescued by Yuri Kusada, a member of the U.S. kayak team, who was working out in rapids nearby, according to Brown. Jones, 24, was treated for minor injuries, Brown said.

He said Jones' lifejacket probably saved him from more serious injury, cushioning his impact against the many rocks in the rapids below Catfish Hole.

The search for the youth also proceeded along the river bank and was assisted by a helicopter, authorities said, before it was called off at 6:30 p.m.

The Catfish Hole area is usually regarded as relatively free of the foaming turbulence that characterizes the Potomac below Great Falls.

Last week's rains and showers made the area much more dangerous, Cabin John Fire Department, Lt. John Harris said earlier this week. Just before last Sunday's drownings, U.S. Park Police had flown overhead in a helicopter, using its loudspeaker to order swimmers out of the water.

Besides yesterday's apparent drowning and the two on Sunday, two Navy seamen drowned May 31 near the Brookmont dam above Chain Bridge in a canoeing accident. That dam, a submerged structure combined with an intake for Washington's drinking water supply, has been described as one of the river's major hazards, dangerous even when the water level is low.