WEEKS BRIDGE, and sites overlooking scenic Charles River--The Head of the Charles Regatta has never pretended to be a purely athletic even in its 25-year history. For all the Erg tests, stadiumstep runs and other sweat-intensive training methods, the autumn culmination of the rowers' Sisyphean efforts is met mostly by that oh-so-collegiate monchalance on the part of lounging spectators along the banks of the dirty water.

From the sidelines, the overriding sense imparted by this series of races is not one of motion, but observation.

"Now why is there such a lull in boats going by?" puzzled one tourist in a bright red ski jacket aloud to his companion, munching on a hunk of Italian sausage. Indeed, very few observers care about who wins or who loses. Very few seem to know that the regatta is a series of races. Some know even less. A woman wearing a Lesley College sweatshirt and sunglasses spoke distractedly, pointing first downstream, then upstream, then across the river, "So what's the deal? The races are going this way?" And others still less--"Are we near Harvard Square?"

Vision, delights to the eyes, the narcissism of seeing and being seen guides those who converge on Boston for the Head. People preen, primp, look, laugh and wave at old friends, newly discovered even if the name ("Cape Cod, right?") escapes. But not too self-conscious, now. Look as if you belong here. Smile!

Even the racers seem to appreciate the smooth aestheticism of surface. The finest crews exude not exertion but fluidity. All move as one. Sculls catch the water as one. Pull through. Slide. Back. Again. Again. And then gone, under the next bridge.

Which boat is which? Shells. Whose oars are those? Sculls. Red and white is Harvard, right? Close enough.

Seagulls circle, as does the biplane dragging the "Smartfood" banner, catching pieces of the brisk morning wind. The wind dies down later, giving the rowers an easier time.

THE Head's party atmosphere has died down somewhat from past years as well. This year at least, a Saturday night reveler would have had an easier time getting into heaven than into Eliot House with its guest list for visitors. Security stayed tight the day of the regatta--helping explain a mellow mood that was cooled further by the weather--although a crack member of Cambridge's "Alcohol Disposal unit" (unmounted division) said the police did not seize as much booze from students as in the past. Last year, bottles, cans and contraband milk cartons filled two dumpsters.

"They're much more discrete this year," said the students' temperance monitor. They're much craftier as well.

"We got a sofa with Budweisers in the bottom. We confiscated the whole sofa," he said, adding that they also nabbed three kegs of beer that been buried in the banks of the river late Saturday night.

"Our [first] year there were kegs everywhere," one senior said to her visiting friend. No more.

A marketing representative for a New Jersey advertiser to college students openly mourned the end of Head of the Charles revelry, though he said the Head still had its strong points. "A lot of blonde girls," he said, "unseasonably blonde." And cleancut young men to match. The perfect J. Crew cover.

The marketing rep, Lehigh U. class of '88, smiled approvingly at a group called "Acoustic Shock Wave" that strummed away at old Crosby, Stills, Nash and Young hits. "There should be more live bands," he said.

And fewer Yuppies with dogs and baby strollers. One professional carrying a perfectly appropriate bottle of California white zinfandel said to her colleague of a newly made acquaintance, "I didn't give him a card, though. What a faux pas."

The other replied, "Well, I only brought two business cards with me."