Potomac Riverfest, Washington's four-year-old annual celebration of the city's revitalized waterfront, opens today as the city's biggest street festival, more popular than its organizers ever dreamed.

The city-spawned event, which drew more than a million people last year, has become a colorful and spirited paean to the Potomac, once so shamefully polluted that dead fish floated in the Tidal Basin, live fish were too contaminated to eat and swimming was taboo.

Today, after a $1 billion investment in sewage treatment plants and other cleanup efforts, the river's recovery is such that this weekend's Riverfest program includes a fishing derby, air/sea rescue demonstrations, boat tours and parades, a boat race, windsurfing and other water sports.

On land, the celebration, which is being held along the Southwest and Georgetown waterfronts this weekend and moves to Anacostia Park next weekend, features a wide range of musical and other free entertainment, a parade, arts and crafts demonstrations, food and drink, boat builders, special children's activities and a fireworks display.

Waterfront festivals such as Baltimore's Fells Point and Inner Harbor celebrations and California's Great Monterey Squid Festival have been popular in other cities for several years. But Washington, according to Beverly Bandler, a spokeswoman for the Interstate Commission on the Potomac River and a member of Riverfest's executive planning committee, "has only recently discovered the river."

By all accounts, including his own, the idea for Riverfest originated with Mayor Marion Barry. He says that both the event and the name came to him in 1984, after an upbeat state-of-the-river meeting with the governors of Maryland and Virginia.

"I like the water, I like fishing and boating -- I'm a Pisces," Barry said this week during a kickoff dinner for Riverfest at Hogate's on the Southwest waterfront. "It's a good family event and the boaters and the merchants love it."

The first Riverfest celebration was put together in a hurry because Barry was eager to disabuse citizens of the notion that the Potomac was still smelly, muddy and filthy. Organized in just a few months, the 1984 event cost about $150,000 and attracted more than 250,000 people.

This year's Riverfest will cost about $400,000 to stage and is expected to attract up to a million visitors. About half of the cost is paid by the city, with the other half funded by private donations.

Police are advising those who plan to attend to use Metro wherever possible because parking will be prohibited along Water Street and Maine Avenue SW. They suggest that motorists park at Metro's Stadium-Armory station lot at 19th and East Capitol streets, or other "outer fringe" lots, and take Metro to the L'Enfant Plaza station, a short walk from the waterfront. Motorists are advised to avoid Constitution Avenue between Seventh and 17th streets NW, the route for today's Riverfest parade, which gets under way at 11 a.m.

Since the beginning, the city's Recreation and Public Works departments have spearheaded the planning for Riverfest, with help from the National Park Service, national and local corporations, merchants and a goodly number of volunteers.

Last year the event spanned five days over four weekends. This year, in deference to complaints from Southwest residents, it has been scaled back somewhat to four days over two weekends.

"It's very peaceful overall, given the size of the event, but there was this feeling that it was too much of a good thing," said Simon Gottlieb, executive director of Advisory Neighborhood Commission 2-D, which encompasses the Southwest waterfront.

Gottlieb and other Southwest residents, espeically those who live in boats moored at the marina, liken Riverfest to Adams-Morgan Day, the popular Northwest neighborhood street fair. Adams-Morgan Day was almost discontinued this year because of concerns that it had grown too disruptive, drawing more visitors than the area could handle safely .

The noise, litter and traffic congestion now associated with Riverfest have made more than a few waterfront residents a tad testy. Many boat owners in the marina clear out for the weekend; last year, one who didn't set sail led other boat owners in turning on their foghorns to protest the loud music blaring from a radio station broadcasting from a Riverfest stage.

Another Southwest resident, Bill Kingston, who lives in the Tiber Island complex at Sixth and M streets SW, is a bit more gracious: "We think the riverfront is a great place to live, and we are happy to share it with everyone -- one weekend a year."

Harriet Epstein, national director of sales for Hogate's, said the restaurant is a strong supporter of Riverfest even though it actually loses business from regular patrons that weekend because of parking problems. "The area is too small to warrant a major celebration of this type, but it's good publicity for the rest of the year."

In Southeast, according to Howard Gasaway, a member of the Anacostia River-based Seafarers Boat Club, residents look forward to Riverfest.

Anacostia Park is far roomier than the Southwest waterfront. Also, he said, it keeps attention focused on the need to clean up the badly polluted Anacostia River, where restoration measures are only now being instituted.

Mayor Barry, who has reaped his own share of publicity via Riverfest, said most waterfront residents "are good citizens" who are "willing to sacrifice a little bit" to promote the beauty of the riverfront. A few others, he said, "are myopic" and don't appreciate what a boost such a hometown event gives the District.

"Most cities would love to have a celebration like ours on the water -- just love it," he said.