The sole survivor of a string of dams once proposed to harness water for the Washington metropolitan area was officially dedicated here today, a $173.4 million structure 50 miles upriver from the nation's capital.

Gleeful officials said the completed dam and its 5.5 mile serpentine reservoir nicknamed "The Blue Dragon" substantially should solve Washington's water supply problems well into the next century.

Many of these same officials earlier had advocated the construction of up to 17 dams in the Potomac basin, but opposition from environmentalists and citizens of the affected areas helped to delay and ultimately block all but Bloomington.

"It's damn embarrassing for me to say that what I was recommending six years ago was stupid," said Robert McGarry, general manager of the Washington Suburban Sanitary Commission and former Baltimore District Engineer for the Army Corps of Engineers. "The key is we got smarter."

What changed the minds of the experts was a combination of factors, including less metropolitan growth than expected, new proposals to tap smaller reservoirs on other Maryland and Virginia rivers and increased regional cooperation on area water problems.

The new dam at Bloomington, in addition to stockpiling a water supply, is expected to serve as a tourist attraction, with a campground and picnic area planned to draw visitors to the 11-billion gallon lake. Already, a boat ramp is in place.

There will be no swimming and no fishing in Bloomington Lake. The Potomac, polluted by acid seeping from abandoned mines, stings the eyes and kills aquatic life as far downriver as Cumberland.

With the ability to regulate the river flow will come some improvement in water quality below the dam, along with a $7 million annual reduction in flood damage. "Instead of a hot shot of acid coming down every three months and burning the bank, we'll bleed it off a little at a time," explained reservoir manager N. Russell Newman.

By the time the water completes its seven-day trip to the Washington area, the acid will be naturally neutralized and, because of the more reliable and regular flow, downriver treatment plants "won't have to keep so many chemicals on site to account for a sudden surge, to keep the water pure," Newman said.

Bloomington stands on a rocky ridge so remote from the millions it is designed to serve that dignitaries were delivered to today's dedication by helicopter.

The dam, third largest east of the Mississippi, rises 296 feet from the original river bed. It consists of an embankment containing 10 million cubic yards of earth and rock delivered down a mountainside by a half-mile long conveyor belt and put in place by hundreds of workers whose efforts proceeded for a decade virtually unnoticed by the outside world.

They moved the Western Maryland Railway from the West Virgina river bank below to 300 feet up the side of Backbone Mountain on the Maryland side, a five-year task thwarted by unexpected earth slides along the 12 miles of relocated road bed. And they levelled the coal and lumber town of Shaw, W.Va., dispersing its 39 families to surrounding communities.

A 1,600 foot tunnel was cut through the embankment to carry the rerouted Potomac inside a 16-foot wide pipe to its normal course below the dam. The tunnel gates were closed July 7 to allow the upstream river to rise beyond the dam. Since then, the water has been rising a foot and a half a day and will reach an eventual depth of 250 feet.

The water is a mere 55 feet deep now behind the huge embankment, just enough to cover the former site of Shaw, not yet enough to cover the treetops now turning autumn colors of yellow, orange and red.

With a mixture of awe and disgust, Victor Kitzmiller goes to the dam overlook every other night to watch the lake fill. He gave up 10 buildings in Shaw for one cramped trailer next to his small barbershop in nearby Elk Garden, W. Va.

"I had a beautiful place down there -- Shangri-la," he said the other day. "But there's no use fighting 'em when they got 500 lawyers on salary. These Army engineers are pretty clever. They think they're gonna have quite a celebration . . . "

Russ Newman commutes 100 miles a day to and from his job here. Most of those employed in the project came from a seven-county area in nearby Maryland, West Virginia and Pennsylvania to satisfy a federal requirement that 85 percent of the work force be local.

Such an arrangement did little, however, for the people living closest to the project. For them, the prosperity promised by the dam project never has materialized.

"There was nobody could run the heavy equipment," said Van R. Pritts, one of five brothers whose Potomac Lumber Co. was moved from Shaw to New Creek, W. Va. Harry and Peter Pritts helped test the topsoil and maintain the trucks at first, but they were the exception.

It was their father who helped the Corps locate a suitable site on the river where water from the future reservoir would not escape through mountains honeycombed with abandoned mines. And that's how the dam got moved six miles upriver from Bloomington, Md., to Shaw, W. Va.

They kept the Bloomington name, of course, because that's how the appropriation went through Congress. "It should be Shaw Reservoir," insisted Van Pritts. "That's the only town that was moved out."

"It was just one of those things," his brother Harry said. "They moved in and moved us out. All we heard about was water for Washington."