Either there are too many beavers in Fairfax County, or there are too many people. Nobody's quite sure which.

Either way, it's become a problem as an abundance of beaver dams this year threaten to flood yards and roads, creating pools of stagnant water that lead to swarms of mosquitoes. The beaver handiwork also threatens the loss of many young trees.

This is no small matter. The Fairfax County Board of Supervisors supported a resolution Monday asking Virginia to provide special help in trapping beavers, which are protected by the state.

"Meanwhile," wrote Mount Vernon Supervisor Sandra Duckworth to her constituents, "we hope you beaver-impacted citizens will be patient with these little busy workers."

There is a large beaver population in Virginia and state officials say they aren't sure if an exceptional number of the sharp-toothed rodents reside in Fairfax, or if the county's dense population simply makes people-beaver cohabitation more stressful.

Margaret M. Becker, who lives on Dogue Creek near Mount Vernon, said beavers have clogged the creek and felled more than 15 trees along the creek bank this spring. "And it's almost impossible to trap a beaver," she said. "They're a very cagey animal."

Near one beaver dam off West Ox Road, the animals sprung a trap with mud, then used the trap in the dam, said Susan Alger, state game warden for Fairfax. Duckworth said she has heard reports of beavers using reinforcing bars from local construction sites in their dams.

With or without man-made reinforcement, beaver dams are difficult to dismantle. The county has been forced to use heavy equipment to do the job, and more often than not the beaver launch an immediate crash rebuilding campaign.

Although beavers are not usually aggressive, they are a tough match for property owners. A beaver can gnaw through a three-inch tree trunk in a matter of minutes, and they keep a low profile in heavily populated Fairfax by working at night and building inconspicuous dams.

Officials are not sure of the number of beavers in the county or the state. But with no beaver hunting allowed nine months out of the year and no natural predators in the state, officials say the beaver ranks may be swelling.

"I think we're being beseiged by beavers," said Becker, "and I think something drastic has to be done about them."