Officials from two parks authorities stood before a lily pad-covered wetland at Bull Run Regional Park on Friday to oppose one of the suggested routes for a tri-county parkway, saying that it would bisect the park and have a devastating environmental impact.

"Build the roads where you need to build them, but don't build them through our parks," said Harold L. Strickland, chairman of the Fairfax County Park Authority Board.

William C. Dickinson, chairman of the Northern Virginia Regional Park Authority Board, said his group understands the need to build roads in the fast-growing, traffic-choked area. But, he said, the benefits of this particular route would be outweighed by its impact on the recreational, environmental and cultural resources of an area rich with Civil War history.

The tri-county parkway would be a north-south route connecting Fairfax, Loudoun and Prince William counties, providing more direct access to the Dulles corridor and easing congestion on Route 28 and two-lane roads, such as Route 15.

The Virginia Department of Transportation (VDOT) proposed three possible routes in April, each approximately 10 miles long. Cost estimates range from $177 million to $548 million.

Two of the routes would link Route 50 in Loudoun and Interstate 66 in Prince William, veering west of Manassas National Battlefield Park. A third -- the one the park authorities oppose -- would swing east of the park, stretching south to Manassas. This route is known as the Comprehensive Plan Alternative, because all three counties have included a parkway similar to this route in their own land-use plans.

The Comprehensive Plan is favored by some transportation advocates and business groups, including area chambers of commerce, because it would accommodate the most cars and would be the only route to run through all three counties and provide direct access to Manassas. Opponents say it is the most expensive and would have the most environmental impact. It would bisect Bull Run Regional Park, which is just east of the battlefield site and across the Fairfax County line in Centreville, and Sully Woodlands, a 4,000-acre parcel the Fairfax parks board hopes to develop into a park.

"Certainly any route you take has an environmental impact," said Paul A. Gilbert, executive director for the Northern Virginia Regional Park Authority Board. "It's a matter of making a choice about which has the least effect."

Charles Smith, senior natural resource specialist at the Fairfax County Park Authority, said the Comprehensive Plan route would run through a large floodplain for nearby Bull Run, Cub Run and Flat Branch that acts as a filter for pollutants. Those pollutants would otherwise run back into the streams that feed the Occoquan Reservoir. The reservoir supplies drinking water to many people in the region. Smith also expressed concern about runoff of oil and gas.

He said the forests and wetlands through which the parkway would run are home to wood frogs and toads, coyotes, spotted salamanders, bobcats and gray foxes, all of which could be threatened by the new road.

Some people who attended the event wore buttons that said "Save the Bluebells," referring to flowers that cover the fields in the spring as the waters subside, drawing tourists. Some are concerned the bluebells could be paved over.

There is no funding earmarked for the parkway, and earliest estimates say it would not be built until 2012. The Commonwealth Transportation Board is expected to decide this fall which route the state will pursue.

Paul A. Gilbert of the Northern Virginia Regional Park Authority Board illustrates the parkway's environmental impact.