The Friends of Dyke Marsh, a group dedicated to the preservation of more than 450 acres of wetlands along the Potomac River near Alexandria, met with a National Park Service official last week to discuss the practical side of their 10-year passion for the marsh.

National Park Service Ranger Charles Mayo, who works at Dyke Marsh, told the group of bird watchers, environmentalists and nature lovers that the park service would install more signs that identify the park as a protected wildlife and wetlands area, as well as other signs that mark bird-nesting areas.

Chairman of the Friends of Dyke Marsh Edward Eder wants the park's signs and some future landscaping at the entrance "to alert people that this is a special area."

The marsh is the last of the metropolitan area's tidal wetlands, unique because its partly saline waters support plants that don't grow anywhere else.

The Friends of Dyke Marsh are worried about the effect of road work in the area. Because of the current widening and upgrading of the Mount Vernon Memorial Parkway, some of the marsh will be destroyed, so the Park Service plans to create an equal amount of wetlands by lowering part of a small island at the end of an old haul road, Mayo said.

The narrow road, which lets visitors enter into the center of Dyke Marsh, was once used by a company that dredged gravel from the marsh bed.

Although the Friends of Dyke Marsh would like more marshland, there are clearly other overriding concerns.

"I trust all truck traffic will be other than in nesting season?" one worried member asked, referring both to the birds and to the necessary evil of allowing trucks down the haul road to take out the extra dirt to lower the island.

Mayo reassured him that only small trucks would be used in that project.

The group has also proposed plans for installing two culverts beneath the haul road to allow the water to flow more freely between northern and southern sections of the marsh.

Dyke Marsh was named for the dikes that settlers during the early 1800s built around the marsh to reclaim it for farming. Apparently these efforts at agrarian ingenuity were unsuccessful. Little remains of the dikes except the antiquated spelling, from which the park takes its name.

An act of Congress in 1959 set aside Dyke Marsh as it exists today, but it started forming more than 6,000 years ago when the swift currents from nearby Hunting Creek collided with the slower waters of the Potomac, causing sediment and silt to settle near the confluence over the centuries.

The rich soil that resulted now is home to a variety of water plants, which in turn support numerous fish and water birds.

"A diversity of marsh vegetation has created a food chain that includes small fish, shellfish, and vegetation that attracts a diverse group of wild fowl," Eder said.

Ruddy ducks, lesser scaup ducks, canvasback ducks (greatly reduced in the Chesapeake area, he noted) tripped off the tongue of Eder who has studied the marsh for five years.

The marsh is open all year, and its Friends point out that it is like a stage that each season brings on new actors and scenery.

In the summer, the marsh is a field of fuzzy brown cattails and slender-stemmed pickeral weeds with bright purple flowers and leaves shaped like arrowheads.

In January, ducks still abound, and about 25 to 30 great blue herons annually winter at Dyke Marsh.

"During the winter, you can see beavers and muskrats, a red-tailed hawk and a resident great horned owl," Eder said.

The owl -- with a two-foot-long body and a wing span of more than four feet -- is winging its way over two-square-miles of Northern Virginia for the purpose of establishing its nesting territory, Eder said.

At least one subway rider has spotted what may be the owl roosting in a clump of trees next to the rails halfway between the Rosslyn and the Arlington Cemetery subway stations. No reports of its mate have surfaced.

After the Friends of Dyke Marsh and other environmental groups kicked up a fuss to oppose Virginia's planned herbicide drop during the past summer's hydrilla wars, some might be inclined to call them "the friends of hydrilla" as well.

"Hydrilla has been good for the fish and water fowl -- bass can breed there," said the group's vice chairman, Ed Risley.

He said the presence of all the hydrilla was actually a good sign that the Potomac River is now clean enough for it to grow, though he also said they would rather see native vegetation in the river, such as wild rice.

The Friends of Dyke Marsh have long-term plans for a self-guided tour of the marsh, complete with posted trail-side explanations.

"Many people in the metropolitan area have no exposure or knowledge of the importance of wetlands to maintaining wildlife. It reduces pollution. It's a natural water filtration and treatment system," Eder said.

"A self-guided tour will break down the natural bias of people who think that wetlands should be drained and paved," Eder said.