The Potomac River from Great Falls to Little Falls, although calm on its surface, conceals a deadly underside of rough water and swirling rapids known to rescue workers as "the drowning machine."

This year, as in past summers, frustrated Park Service rangers again are compiling the statistics on Potomac River deaths.

Although swimming is prohibited and boaters are warned against entering the river in several places, both swimmers and boaters continue to fall prey to the Potomac.

So far this year, the Great Falls-to-Little Falls stretch of river has claimed five lives, the same number at this time last year, according to District Ranger Michael Brown. Four of this year's victims have drowned on the Maryland side. On Friday, the Virginia side of the river claimed its first casualty, 16-year-old Il Sun Yoon of Fairfax, who was swimming with three friends near Cowhoof Rock when he was swept downstream in the currents.

The latest Maryland victim is believed to be Richard Allen Van Gombos, 23, of Edison, N.J., who disappeared Sunday afternoon after he and a companion capsized their rented canoe after taking it from the canal into the Potomac near Little Falls. His companion swam to shore to retrieve a paddle, Park Service police said, but Van Gombos apparently was pulled under by strong currents as he attempted to right the canoe.

Also missing is Thomas B. Sterman, 19, of Northwest Washington, who was pulled under water while swimming near Great Falls on Sunday. None of the three youths have been found. Sgt. Vincent Scaide of the Cabin John fire and rescue station said that rescue efforts have been suspended for three to five days to wait for bodies to surface.

Underwater rescue teams have been hampered by the swift current. Despite the wearing of heavy weights, divers have been unable to conduct searches for prolonged periods of time, park rangers said. In addition to the divers, the rescuers use helicopter and motorboat patrols to search for drowning victims.

The Potomac is particularly treacherous now because recent rains have increased the speed of the flow, said Warren Isman, chief of the Montgomery County Department of Fire and Rescue Services. "The flow is the highest in the last two and a half years. We need to warn the people."

Ray Fletcher, co-owner of Fletcher's Boat House, said warnings do not always work. "Some young people have to take the challenge," he said. "I don't know why."

Park ranger Brown noted, "Most of the accidents were swimmers who had a greater respect for their abilities than they had for the river. Running water, moving water, is extremely powerful."

The problem, said police and park service rangers, is the river's calm surface. "It looks very peaceful, but underneath there is a deadly current," said Park Service spokeswoman Sandra Alley. "Once you go in, your chances of getting out are very slim."

From its elevated starting point near Thomas, W. Va., the river's 83-mile journey to Point Lookout is marked by rocks and dams. The Potomac's center is its most dangerous point, with a whirlpool-like pull that makes life-vests virtually useless. "Even if you have a life jacket in the center of the river, you can still drown," Brown said.

The annual body count has caused frustration among police and rescue workers. They have erected warning signs, which are ignored. They regularly patrol the waters looking for swimmers. Boating is not prohibited on the river, although Fletcher Boat House makes its customers sign contracts pledging not to take their boats from the canal into the river.

"It appears the only way to really keep people out of the river is to line park rangers along the river banks and have them hold hands," said Fletcher.

"The biggest thing is education," said Ranger Brown. "Every year we have the same kind of thing, because kids don't realize this can happen."