Hai Van Huynh, a 19-year-old junior at Arlington's Wakefield High School, scrambled down through the trees and out onto a ledge at Great Falls National Park, overlooking a chute of the Potomac River known as "The Spout."

He had brought his fishing rod and about dinnertime Monday, he hooked a big one. But, as he was trying to reel it in, he slipped and fell into the rushing river. The last thing his companions remember is Huynh's hand sticking out of the water.

Huynh, a Vietnamese refugee who had arrived in America eight months ago, is missing and presumed drowned, the fifth person to have been claimed by the Potomac so far this year.

His accident comes at a time when Park Service officials are gearing up for their annual battle against sunbathers and fishermen who insist on climbing the Potomac's picturesque and craggy rocks, despite the many lives lost in the river each year.

It's a deceptively calm-looking river in some places, with shady pools, smooth water and low dams. The 11-mile stretch between Seneca and the Chain Bridge can be treacherous, and it averages five to seven accidental drownings a year. Last year, 14 persons drowned.

"This river is unforgiving," said Joan Anzelmo, the Great Falls Park site manager. "You have to respect the power. It has strong currents and really powerful undertows."

"The hydraulics of the river are such that it is like a vortex," said Howard E. McCurdy, an American University government professor who has studied the Potomac. "There are places that simply pull you under," he said. "If you don't have a life vest, you can't swim out of them."

McCurdy said he has canoed Mather Gorge, where Monday's accident occurred, when the river was at similar levels, and " . . . the canoe was turned around and sucked down into the water."

Despite many large warning signs in the parks along the river and the possibility of a $500 fine and up to six months in jail for entering the river there, many of the 5,000 to 6,000 people who visit the park each weekend day persist in wading, swimming or what the rangers call "rock-hopping."

At midday yesterday, for instance, two men had hopped from rock to rock to reach a sunbathing spot far into the river, just above the chute where Huynh disappeared.

"That's really dangerous where they are," said Anzelmo, watching from shore. "If one of them fell in, they'd have to be lucky to be able to grab a rock. Probably, the currents would sweep them up, and they'd come down into the chute. They wouldn't survive."

There is no one particularly bad stretch of the river as it winds from Seneca to Chain Bridge, although some parts are rougher than others. "It's the whole thing," McCurdy said. "You're talking about a whitewater river." He added that parts are worse when the water is low, and that others become especially dangerous when it is high.

The problem, he said, is that people usually respect the river when it is full and raging. When the water drops, the Potomac appears less threatening.

The major cause of accidents, officials say, is carelessness. Alcohol is thought to be a contributing factor, Anzelmo said, although drinking is not allowed on park premises.

Last week, Reps. Frank R. Wolf (R-Va.) and Michael Barnes (D-Md.) urged Interior Secretary Donald P. Hodel to impose automatic $200 fines for persons who enter the river illegally from national parks on either side. Although violators can now be fined $500, the usual punishment is $25, officials said.

The two congressmen also have announced that a $3.5 million plan for modifying the river's Little Falls Dam has been approved by the Army Corps of Engineers and the District of Columbia. The plan will eliminate the dam's dreaded hydraulic roller effect, which some call the "drowning machine" because of the powerful and dangerous currents it creates. Huynh's fishing companions -- An Ngoc Le, a 26-year-old hotel worker and Loc Thanh Le, 16, a 10th grader at Wakefield High -- could not be reached yesterday.

Huynh's brother, Long Thanh Le, 14, said through a translator that his brother and the two friends had gone to Great Falls because they loved to fish for carp, bass and catfish, and they had seen others fishing from the rocks along Mather Gorge.

Le said he doubted the men were able to read the warning signs on the cliff. They had escaped Vietnam in 1982, spent two years in Thailand and came to the United States only last August, he said.

"Probably," he said, "they just didn't know."