There is nothing else in Northern Virginia like Huntley Meadows Park, a wilderness amid subdivisions just off the fast-food jungle of Fairfax County's Route 1.

The 1,261-acre park, the largest run by Fairfax County, is a rich ecosystem that is home to deer, raccoon, beaver, fox, river otter and 200 kinds of birds, including relatively uncommon varieties such as the yellow-crowned night heron, pied-billed grebe and least bittern.

Bluegill, bass and crayfish can be found in the shallow waters of the park's marsh -- good eating for the blue heron, egret and other water birds that stop by or nest in the area's largest protected wetland.

"You see species there that you don't see in a normal forest," said Sue Becker, president of Friends of Huntley Meadows, whose 250 members work on park programs and preservation activities. "You get to see animals you don't see otherwise."

The park has acres of field and forest, but its centerpiece is a 100-acre marsh buzzing with beavers and birds and crisscrossed by boardwalks, with an observation tower at one end. In summer, the swamp rose, buttonbush and lizard's tail create a pink and white tapestry of blooms. Now, with winter approaching, the flowers have faded, but the trees edging the marsh are beginning to flame with color.

The park is only 10 miles as the crow flies from the U.S. Capitol, slightly longer by paved road, and is so vast that three novice hikers who left the marked trails were lost there overnight last year. The park's locale and emptiness entangle it in problems at the extremes of American life: The staff has apprehended people poaching deer as well as homeless people seeking shelter.

Huntley Meadows has none of the usual features of a county park -- no chin-up bars, picnic tables, hot dog stands or ball fields are here, and never will be under its master plan. Except for the boardwalk, newly constructed visitor center and five trails near the entrance, nature is in charge. Visitors are not barred from the more remote parts of the park, but park officials hope they won't go there so those areas will be free of human influence. About 98 percent of the park's 200,000 visitors a year comply.

Although the park meadows are groomed to keep them from turning into forest, the grass near the entrance gate is untrimmed, the better to accommodate wildlife. When trees fall, they are not cleared unless they block a path.

The park's most dramatic concession to nature, however, is in leaving Huntley Meadow's beavers alone.

"We sometimes consider them to be a part of the staff," said Gary Roisum, the park manager.

The beavers' lodges and dams have raised the water level of the marsh by a foot and a half in the past decade. The wetlands acreage of Huntley Meadows is growing -- now numbering more than 800 acres -- and the beavers are a big reason.

The higher water level is making the boardwalk unsafe, and rather than regulate the beavers, the county will replace the boardwalk. Park officials will spend $1.17 million in the next four years, raised through a 1988 county bond issue, to rebuild the boardwalk, improve trails and study Huntley Meadows' health and condition.

The beavers are transforming the park in other ways. As the marsh enlarges, it drowns the roots of trees around its edge. Now, between the forest and the swamp, there is a thin row of bare, limbless snags.

"We look at this as a sacrifice of one life for other lives," Roisum explained. "These snags provide nesting cavities for other wildlife, particularly songbirds."

Roisum has been on the staff of Huntley Meadows since 1978, three years after Fairfax County acquired it from the federal government. Before that, it had been a secret Navy test site, a farm and a plantation owned by the family of Virginia historical figure George Mason IV.

Roisum, the three other full-time staff members and a corps of volunteers run 400 programs a year -- mainly school visits, walks and lectures with an environmental theme. Their basic message, said Roisum, is that wetlands are not just swamps, but "natural wonderlands" that are valuable breeding grounds, pollutant filters and habitats. He used to have to argue harder for this point of view, but he said attitudes have changed in this ecology-conscious decade.

The biggest threat to the park, Roisum said, comes not from the visitors inside but from activity outside. Sediment that runs into streams from nearby development, especially when developers take inadequate precautions during construction, can drown the smaller fish and insects that live in the water. During one big storm four years ago, five inches of silt washed into Dogue Creek at the park's western border.

Still unresolved is the fate of a road that environmentalists say could destroy Huntley Meadows. Park staff members are officially neutral on the proposed Lockheed-Van Dorn connector, which would cut through the northern edge of the park, but many of its volunteers are against it.

The 2.2-mile road would link Route 1 and the Springfield Mall area, and county officials say it is needed to resolve long-standing traffic bottlenecks.

The new Huntley Meadows visitor center, which has a 75-seat auditorium and larger parking area, is expected to open in the next few weeks, replacing a smaller facility that opened in 1983. A wetlands diorama exhibit, complete with stuffed waterfowl in flight and photo-murals, will be installed by next spring.

The park's supporters are proud of the visitor center, but uneasy that it may bring more people than Huntley Meadows can handle. On Sunday afternoons, Becker said, "you have to elbow your way across the boardwalk . . . . Sometimes it's nice to have it to yourself."

"We're just torn," said Norma Hoffman, an active volunteer who founded a citizens group to fight the proposed Lockheed-Van Dorn connector. "We're afraid with more exposure, we'll get more requests {for programs}. Like the Appalachian Trail, overuse can be a problem."

Main entrance: Lockheed Boulevard and Harrison Lane.

Hours: Park open dawn to dark. Visitor center temporarily closed.

Phone: 768-2525.

Best wildlife viewing times: Early morning and evening.

Admission: Free.