The Army Corps of Engineers shut off a 3,000 stretch of the Potomac River yesterday, an action that may forestall a serious water shortage in the Washington metropolitan area in the 1980s.

With the aid of three bulldozers and a temporary earthern dam, the engineers diverted the river from its rockey bed 180 miles northwest of Washington and into a 1,619-foot long tunnel around a mountain gorge.

The action will enable the corps to complete construction of what will be the first - and probably last - dam on the river. Creation of the Bloomington Lake, as the 5 1/2-mile-long reservoir behind the dam will be called, will give the metropolitan Washington area enough water to meet the area's growth needs for five to 10 years, engineers say.

That will postpone until the 1980s what some planners say will be a critical water shortage in the area.

At 11:55 a.m. yesterday, three bulldozers - two of them on the Maryland side of a mountain gorge and one on the West Virginia side - plugged a 75-footwide hole in a temporary rock and earth dam in the riverbed. Fifteen minutes later, the rain-swollen river began to rush into the nearby 16-foot wide tunnel, cut into the side of a mountain.

A permanent dam to be built downstream from the pemporary dam will create a 1,000-acre recreational lake that engineers say problably will be barren of aquatic life because of acid seeping from abandoned coal mines in the West Virginia mountains.

When Bloomington is finished, sometime in early 1981 according to the corps' schedule, it will be able to supply a minimum 135 gallons of water a day to the Washington area during the summer months and, according to another estimate, as much as 200 million gallons a day.

By the time the water gets downstream to Washington, the acids from the mines will have diluted to the point that they pose no health hazard.

The dam, with a capacity of 42 billion gallons, will be used to increase the Potomac's flow during the dry summer months. Without the reservoir, the Washington area could encounter serious supply problems in the early 1980s, according to various projections.

Yesterday's diversion, which had been postponed twice - once because of heavy rains, another time because of a one-day wildcat walking by construction workers - went smoothly. Fifteen minutes after the river entered the tunnel, it came rushing through an outlet around a bend in the river.

"The first thing that came out of the tunnel was a Pepsi-Cola can," said N. Russel Newman, the corps' assistant project engineer.

It will take the corps about 2 1/2 years to fill the gorge with 10 million cubic yards of dirt and rock for the permanent dam. "All three of Egypt's great pyramids at Giza could be put in the gorge, and it still wouldn't be filled up," s Corps official said.

Bloomington will cost $153 million when it is finished. According to Daniel Sheer, a planning engineer who is studying the area's water supply problems for the corps, the "the least cost-effective of 16 potential dam sites on the Potomac." But, Sheer said, "because none of the other dams could be built, Bloomington was cost-effective."

The other dams were not built because of opposition by local residents and environmentalists who objected to the flooding of thousands of acres of picturesque countryside.

Bloomington survived because local opposition was insignificant - the impoundment will control periodic flooding between Keyser, W.Va., and Cumberland, Md. - and because it was planned before the word dam became a battle cry for aroused environmentalists.

When it was conceived almost 20 years ago, one of Bloomington's main purposes was to help dilute sewage effluent that was being discharged into the river. But with more advanced treatment producing a cleaner effluent, the project's emphasis has changed. Now a major purpose is considered to be supplying additional water to the growing Washington area.

According to Sheer, the "Bloomington on the line, there should be enough water in storage to supply the Washington metropolitan area's demands well into the first quater of the next century, but only if the region takes steps to combine its water systems.

With interconnections as well as Bloomington, Sheer said, the region can stretch and usefulness of existing reservoirs, and thus head off the possibility of local or even regional water rationing.