A crowd gathered around a rented blue dumpster at Alexandria's Belle Haven Marina yesterday and watched as the Army Corps of Engineers launched its first major attack against hydrilla -- the underwater weed that has infested parts of the Potomac River.

In the first of four experiments at using mehanical means to control the weed, a 37-foot-long aquatic harvester resembling a lawn mower swept through the water with paddle wheels churning and mower blades snipping.

Within minutes, the machine had accumulated so much of the innocuous-looking green stuff that owner-operator Bob Dabinett had to steer his rig back to shore and dump his haul.

Two men with pitchforks transferred the weed to a conveyor belt that fed into the blue dumpster.

"They're going to weigh each dumpster load and then transport it to a landfill in Lorton," said Rep. Stan Parris (R-Va.), who was at the marina with a handful of press releases.

Until recently, the corps had planned to use the controversial herbicide Diquat on a one-acre test site at the marina. But seasonal temperature changes affecting growth of the weed, combined with Virginia state officials' reluctance to approve use of the herbicide -- which some environmentalists vigorously oppose -- caused the corps to opt for mechanical control measures.

"If the state hadn't delayed so long on its response, we wouldn't have to do it this way," Parris said yesterday as he watched the tedious harvesting process.

"Diquat has already been approved for use in free-flowing waters by the state in general," he said.

"Diquat would have been much cheaper. The amount they were going to use was insignificant."

Corps officials said the harvester will work at the marina for two days and probably clear one to two acres of hydrilla in that time. Dabinett, who has a corps contract for $73 an hour, will then move with his Mud-Cat harvester upriver to the Old Town Yacht Basin.

"No matter what they do, they're going to have to do it again," said Frank Dawson from Maryland's Coastal Resources Division. corps officials said the harvesting would have to be repeated next spring when the weed grows back.