The Army Corps of Engineers turned on a mammoth vacuum in the Potomac River yesterday, hoping to rid the river of hydrilla, that rapidly multiplying plant that some say threatens to choke off the river.

"Hydrilla is growing so quickly that it could soon infest most of the Potomac. It's a very serious problem," Col. Martin W. Walsh said yesterday at Alexandria's Belle Haven Marina.

The plant already has affected recreational boating activities at the marina, according to corps officials.

The plant, which looks like an ordinary green garden weed, forms impenetrable mats just below the surface of the water, clogging river access, collecting debris, choking vegetation and diminishing swimming and fishing activities, according to watermen and biologists in Florida and other areas where hydrilla has been a problem.

"If we don't control this thing, the economic impact could be well into the millions," said Rep. Stan Parris (R-Va.), who was at the marina to witness the demonstration of what is called a "diver dredge," which resembles a large vacuum cleaner with its two hoses connected to a raft. A diver takes each hose down to scoop up the hydrilla.

The dredge will cost $15,000 to use this week. "It's expensive, but we have to study all options," Walsh said. "The problem is significant now, but it's going to be explosive in the future."

By 1995, the corps estimates that hydrilla, which now infests 600 acres of the Potomac, will cover more than 34,000 river acres between Chain Bridge and the Rte. 301 bridge.

But some biologists, including Virginia Carter of the U.S. Geological Survey in Reston, aren't convinced that hydrilla threatens the life and use of the Potomac.

Carter has said that she is not sure hydrilla will grow at as rapid a pace here as it has elsewhere.

Carter and those who support her view also say they are concerned that in eradicating hydrilla -- either by herbicide or dredge -- the corps will destroy other vital submerged grasses, which have recently made a comeback in the Potomac.

Parris said yesterday that he was concerned that Maryland and Virginia had ruled out using herbicides in the river.

Diquat, the chemical that the corps planned to test on hydrilla, is approved by the Environmental Protection Agency for use in fast-flowing waterways, but Maryland and Virginia state officials decided against allowing the corps to use it last year.

"They are going to have to be willing to bite the bullet and pay for the cost [of mechanical harvesting] if they don't want to use Diquat," Parris said.

The corps, which must "provide for control and eradication" of such problems as hydrilla, is preparing a master control plan scheduled to be released next year.

"We just think it's premature to use herbicide," Richard L. Cook, a deputy secretary for the Virginia Department of Commerce and Resources.

Cook said Maryland, Virginia and District officials plan to meet July 17 to discuss the best approach for a coordinated attack on hydrilla.