Four years ago on the banks of the Potomac River near the Watergate stood a sign that bespoke a city's jaundiced attitude about its finest natural resource.

The sign read: "POLLUTED WATER; NO SWIMMING; FISH CONTAIMINATED." It was place there by the National Park Service, which had no known scientific justification for installing it, other than a gut feeling that it would be better if people didn't fish or engage in water contact spots there.

The sign disappeared after a series of newspaper articles in 1977. And a week and a half from today, on May 13, U.S. Fish and Wildlife Service biologists will man a fish-checking station on Park Service land 100 yards from where the sign stood.

The checking station will be one of five set up for a five-day celebration called "Potomac Awakening." The biologists will be weighing and measuring anglers' catches in a river fishing derby sponsored by government and community organizations and private businesses from around Washington. When they are through weighting and measuring, the scientists will turn the fish back to their owners and wish them happy eating.

It marks a startling turnaround in the governmental perception of the quality of Washington's river. Potomac Awakening, according to organizer Paul Leach of the National Marine Fisheries Service, is designed to "reacquaint the people with our reborn river. It's a celebration of this great resource."

It's a celebration not without a certain irony. The general view of the quality of the river has soared in the last 10 years as a result of the effects of new federal laws on sewage effuents. A decade ago the river, oxygenstarved and loaded with nutrients, turned to disgusting pea soup color and consistency in the summertime. That doesn't happen anymore, now that villages and towns upstream have sewage treatment facilities and the District has its giant Blue Plains plant.

But last month a wall collapsed at Blue Plains, knocking out the chlorination function and sending undisinfected effluent into the river for four weeks. It was a reminder of the bad old days but, according to D.C. water quality experts, the effluent still was getting substantially treated and the mishap never threatened the Potomac Awakening.

Good, because this is an event long overdue. Ask the people at Swain's Lock, Fletcher's Boathouse, the Maine Avenue fish market and elsewhere on the river what the most common question is in their daily business and they will say it is people asking, "Can you actually eat fish from the river?" t

People have eaten fish from the river for years, but only recently have they done so with the positive approval of District officials. One of the first problems the Potomac Awakening planners encountered was lining up sponsors for the fishing derby.

"The first thing prospective backers wanted to know," said Jim Racine of the Interstate Commission on the Potomac River Basin, "was, 'Are people going to get sick when they take their fish home and eat them?"

ICPRB asked for and got assurances from both Maryland and Washington. District officials, it turned out, even collected catfish samples from four points on the river, ground them up whole and tested them for heavy metals and chemical pollutants, which are the worst dangers of polluted waters. mThe fish passed with flying colors.

The one cautionary note city officials inserted was that all fish from the river should be cooked adequately before serving. Bacteria and viruses are common in river water, particularly where there is some sewage effluent, no matter how well treated it is. Cooking kills bacteria and veruses. "I would strenuously advise against eating raw fish from any river," said Jim Collier, chief of the water hygiene division of the District's Bureau of Air and Water Quality.

His agency's view of the edibility of Potamac River fish changed in 1978 with a revised version of the law banning swimming in the river. "The new law specifically exempted fishing and boating from prohitited activities," Collier said. "We assume that if we allow people to fish in the river they're going to take the fish they catch home and eat them. We know of no reason why they shouldn't, as long as the fish are thoroughly cooked and were properly handled prior to cooking."

So (it is hoped) die a myth about our river.

Potomac Awakening is sponsored by a 200-member group called Washington Area Waterfront Action Group. It is in two parts -- the five-day free fishing derby and three days of exhibitis, displays, music and programs at Constitution Lake on the Mall.

The derby runs May 13-17 and will include daily prizes for biggest fish in seven categories for both adults and children 12 and under, plus a grand prize for biggest fish of all and trophies for biggest fish overall in each category.

There are five checking stations: the Tidal Basin near the paddleboat rentals; Washington Sailing Marina at the concession stand; the tip of Hains Point; Thompson's Boat Center near Watergate, and Fletcher's Boathouse between Chain and Key bridges. Station hours are noon-7 p.m. May 13-15, 9 p.m.-7 p.m. May 16 and 9-noon May 17. Rental tackle and bait will be available at each station.

The fishing derby covers waters from the Cabin John Beltway Bridge south to Woodrow Wilson Bridge, plus the Tidal Basin, Washington Channel and C&O Canal. Eligible fish species are black bass, carp, catfish, striped bass, perch, sunfish and crappies.

The exhibits and displays at Constitution Lake, on the Mall just north of 17th Street, run noon-7 p.m. May 15, 10-7 May 16 and 10-5 May 17. Some 150 exhibits are slated, including live music by local bands, fish fries and displays by boat dealers, tackle manufacturers, environmental groups, a live marine life "touch tank," a trout pond for kids, scuba divers and commercial fisherman displaying their catches. Admission is free.

The arards for fishing derby winners will be at 3 p.m. Sunday.

If Leach has his way it will become an annual event that someday leads to "Who knows?" he wonders. "Fishing piers on the river? Swimming beaches? A major urban fishing program? More marinas? Places to tie up your boat and stop in for dinner?"

He's hoping.