Great Falls National Park rangers said they prayed it would rain today, not so much to keep people away but to keep them alive.

On each of the past two cloudless Sundays, a person who swam or slipped into the Potomac River near the park died.

If the sun shines as brilliantly as predicted today, drawing as many as 5,000 people to the park, authorities said the chances that another person would drown would be as chilling as the cascading waterfalls themselves.

"There's a 1-in-11 chance that someone drowns today ," said Howard E. McCurdy, an American University professor who has analyzed the 57 drownings that have occurred between Great Falls and Little Falls on the Potomac since 1975 for the National Park Service.

"A sunny, June Sunday when the water level is at a medium height makes the park service very nervous," McCurdy said Friday. "These tragedies aren't as random as we tend to think."

The study explains four factors contributing to a higher probability of drownings: weather, season, day of week and water level.

"Danger days" are what Joan Anzelmo, the park service site manager at Great Falls, calls warm, spring Sundays when the water level is between three and five feet.

On these days, when visitors sunbathe, climb rocks and picnic along the park's shoreline, Anzelmo said, the risk of drowning peaks, particularly because the water level is not high enough to alert people to the river's deadly undercurrents.

There are only five park rangers and two park police officers on each side of the 11-mile stretch of river between Great Falls and Little Falls in Virginia and Maryland.

For Earl V. Kittleman, the chief National Park spokesman, that means "I'm praying for clouds, drizzle or rain . . . , anything until we get past June weekends."

Anzelmo said that by July and August, news of the annual spring drownings scares more people away from the slippery rocks edging the Potomac, and fewer people visit Great Falls because many spend weekends at the beach or vacation elsewhere.

Yesterday, despite posted warnings and widespread news reports of the six drownings that have occurred this year, dozens of Great Falls patrons climbed perilously close to the river's edge.

Paul Galison, a 16-year-old Langley High School sophomore, stood a few yards from a sheer 50-foot drop into the Potomac yesterday. "I know it's dangerous," he said. "But the water is hypnotic . . . . I love to climb on these rocks, and I can't help but want to dive in."

Charles C. Walbridge, the safety chairman of the American Canoe Association, said Friday that the Potomac, which varies in depth from one to 100 feet, is "potentially one of the most treacherous white-water rivers" in the country.

"Part of the problem," Walbridge said, "is that it runs by an urban park."

"Just like a country person can get fleeced by a cabbie or mugged in a bad part of town, urban people who don't understand the river can drown," Walbridge explained.

Last Sunday, police airlifted more than two dozen people from river islands when fast-rising water stranded them, covering the rocky paths they had taken from the shore.

That rescue operation -- which Kittleman estimated cost nearly $10,000 -- sparked an outcry from area officials who said violators of the federal law that prohibits people from entering the Potomac River from parkland in order to wade or swim should be heavily fined.

Though the offense carries a maximum penalty of $500 and six months in jail, because of the chaos involved in rescues and the fact that most offenders are young, authorities usually issue no citations or fine violators as little as $25.

That may change, however, now that U.S. Attorney Elsie L. Munsell has requested a court order requiring future violators to appear in court.

Six of the eight Virginia judges whose signatures are needed to issue the order have signed it.

Park police say educating people to the river's deadly currents and undertows is the park service's top priority.

Because three of the six victims who drowned this year were foreign-born, Anzelmo said, the park service is considering soliciting foreign-speaking volunteers to patrol the shores warning visitors of the danger, especially when the weekends are sunny.

"I would go down there if I didn't think I'd get locked up," said Joseph A. Miller, 36, a former Herndon resident living in Atlanta.

"My mother told me this morning to stay away from the water," he said.

"But every time I come back home, it's the first place I come. There's something about the fast-moving water. It makes you want to jump in. I guess it's the thrill, the challenge."