As the last days of summer lure thousands of Washingtonians to the sun-warmed rocks and sycamore-shaded trails of the Potomac shore, Mike Brown fears it is likely that someone will literally die to get into the river.

Numbers, says the district ranger of the Chesapeake and Ohio Canal National Historical Park, don't lie.

Last year two persons drowned in the critical stretch of the Potomac from River Bend Park above Great Falls to Chain Bridge.

This year, in the stretch between River Bend Park and Key Bridge, there have been 14 drownings -- the highest number on record, and nearly three times the yearly average for the past 10 years -- an epidemic of deaths that only threatens to spread during this Labor Day weekend.

There are reasons for the drownings, Brown says, the most prominent being this year's heavy and unusually spaced rainfalls in the river's vast watershed that have kept the Potomac running, even in normally dry and somnolent August, at near-flood levels more common to February or March.

There is also the character of the river itself, which superimposes essentially wilderness dangers -- cliffs, white water, a major waterfall, and cold water temperature (as well as two dangerous dams) -- within 10 miles of a major urban area.

"Urban people know they could get mugged or shot," Brown said, "but they aren't attuned to outdoor environmental dangers. These kids know the river can kill, but when they get down there with their blanket and cooler it doesn't look that dangerous. They have a few beers or some marijuana and they think they can cope with it.

"They slip off the rocks into a pool and start splashing around. And then the current shifts momen- tarily . . . and they're gone."

Ranger Brown and his colleagues in the U.S. Park Police have been struggling for years to cope with the Potomac's toll, even though the river really isn't theirs to patrol. North and south of the District it belongs to the state of Maryland, and through the District is the responsibility of the D.C. Police harbor patrol.

Since the Park Service rangers control the south shore from Great Falls to Mount Vernon, however, and the north shore from the District all the way to Cumberland, Md., they have been left with de facto jurisdiction over part of the river itself -- a factor that grates no small amount on National Park Service budget makers.

The Park Service has chafed for years under the periodic public and political protests that follow each rash of drownings -- drownings they have felt ill equipped, by either jurisdiction or manpower, to prevent.

This weekend, for example, the Park Service will have only 11 persons to patrol the 10 miles of river and 20 miles of shoreline between River Bend Park and Chain Bridge, and most of those will have other duties as well. An additional eight or nine persons will be assigned to other park jurisdictions or duties nearby, but will be available for emergencies.

The problem has been compounded by enormous increases in river visits in the past few years: The number of visits to the C&O Canal park alone, Brown says, has doubled from 3 million to 6 million in the past four years.

In an effort to maximize the effectiveness of the limited number of Park Service rangers, Brown has been working with American University professor Howard McCurdy on the first statistical breakdown of Potomac River drownings ever attempted.

"We have some data over about 15 years, better data over the past eight and very good data over the past four," Brown said. "We are working now to get full coronor's reports from all the surrounding jurisdictions."

"The biggest problem," McCurdy said, "has been simply finding out who drowned. Due to the confused jurisdiction over the river, police reports on the drownings are all over the place. There is no centralized reporting point."

McCurdy, a canoeist and neighbor of Brown in the Brookmont section of Montgomery County, volunteered six hours a day for two months during the summer of 1982, compiling information on which the Park Police could base their patrolling patterns.

Why?

"I got sick of seeing people drown."

According to McCurdy, about a dozen people drowned between River Bend Park above Great Falls and Chain Bridge in 1973, the highest previous year for which he was able to compile records.

The following year, the Park Service rangers increased patrols at Great Falls and "only three or four drowned." The year after that the Park Service formally launched a "River Safety Program" of patrols and warnings from which the present "Visitor Protection Program" evolved.

His study, McCurdy said, covers principally the years 1975 to 1982, when the annual drowning rate ranged from two to eight and averaged about five.

"We found water level to be the greatest single predictor of drownings," McCurdy said, "but it isn't simply a question of high or low being dangerous."

What happens, he and Brown said, is that different levels produce different conditions at different spots along the river.

According to Brown, there are three basic potential victims on the river. They are boaters, rock hoppers and swimmers, and fishermen.

The boaters, principally canoeists and kayakers, are usually white males ranging in age from 18 to 22, Brown said. They used to get in trouble and drown 10 years ago but are "not much of a problem now."

Working with the Canoe Cruisers Association and other boating groups, the Park Service took the educational initiative, including placing extremely specific signs at most launching points to inform those venturing out on the river exactly what they were likely to find and what skills they would need.

Since then, the experience and awareness level on the river has vastly increased, Brown said. There is almost always someone experienced around to accompany novice paddlers, "and often we have boaters who know the river getting someone else out of trouble."

The swimmers and rock hoppers are another matter. Most are younger, about 16 to 18, Brown said, and also predominantly white males, usually partying with an all-male group.

"They're in that age where it's a macho thing," he said. "They have a couple of beers and somebody decides he can swim the river." At least 50 percent of them have been drinking, Brown said, "and we think if we had all the autopsy reports that figure would be higher. They are our principal problem."

Fishermen, Brown said, are usually older and often black or Korean. "They very seldom give us a problem. They usually know the river pretty well and work the section down around Fletcher's Boat House or Chain Bridge. We've had a couple drown this year, but I think that's because with the river up, the currents this summer have been so much stronger around the rocks where they usually stand."

By correlating the locations of past drownings with water levels at the time, Brown and McCurdy have been able to notice how drowning victims congregate at different spots at different times.

For example, high water at Great Falls usually scares would-be swimmers away from the falls there, but then they often try the water around Little Falls Dam, which is nearly always lethal.

Low water at Great Falls, on the other hand, lures rock hoppers from the Virginia side out to the falls' very edge, where a minor slip can send them downstream to their death.

The most dangerous water of all may be water in the middle range, measured at between three and four feet at Little Falls, which Brown says uncovers enough rocks above and below Mather Gorge to tempt the rock hoppers, looks placid, but carries deceptively fierce currents.

Using the statistical pattern, Brown said, Park Service rangers have been able to concentrate patrols at past drowning sites and warn hundreds of potential victims of the river's danger. Their efforts, he said, have cut down drownings at many of those sites, but this year, with the river up, people are drowning elsewhere as well.

Brown won't identify his present areas of greatest concern because "that would just send people somewhere else and the whole river is dangerous at some point or other. Now, at least, we know where people tend to congregate and we can keep an eye on them. We don't have the manpower to patrol every inch of shore."

In addition, Brown said, warnings alone are never enough: "Sometimes the danger itself becomes a challenge."

A case in point is Little Falls Dam, where a peculiar hydraulic effect creates what canoeists and kayakers call "a drowning machine." According to McCurdy, the dam has claimed 40 lives, and is widely known along the river as a particularly lethal hazard. Yet it still lures people to their death.

Last May, when five persons, four of them soldiers from Fort Myer, drowned attempting to raft over the dam, Brown said, "four of the seven in that boat had run the dam the week before and been warned twice then that the dam was a killer."

A caretaker on Sycamore Island warned them before they reached the dam, and when they emerged from the water later at Fletcher's Boat House, "Joe Fletcher himself told them they could have been killed. But they went back, got a bigger boat, got three more friends and came back to do it again. And five of them died. I don't know what we can do about a situation like that."

What the Park Service rangers have done, Brown said, "is get very strict about drinking and drug use" at Great Falls Park and along the C&O Canal, to undercut the beery bravado that seems to figure in so many of the drownings.

Last weekend, for example, by employing their statistical pattern, patrols discovered two groups of youths swimming the river, one from the Virginia side and one from the Canal side, just below Mather Gorge.

Both groups had coolers of beer and had been drinking, Brown said, and those on the Canal side had a bag of marijuana. The youths became disorderly and resisted arrest and ultimately were arrested for drinking and booked in Montgomery County, Brown said.

None of them would believe they were in danger, Brown said, but then that's not unusual. "They were just having fun. It's a beautiful place and the idea of death in that setting is very foreign to them.

"But how many kids do you know who have any concept of death or serious injury? At that age, they all think they're invincible.