A 42-mile stretch of the Potomac that environmentalists say is threatened by residential development, cell towers and talk of another bridge over the river is being designated a "Last Chance Landscape" today by a national preservation group.

Washington-based Scenic America annually selects what it believes are the nation's 10 most challenged scenic landscapes -- sites thought to face imminent and potentially irrevocable harm. This year's picks include the Middle Potomac scenic corridor, stretching from Point of Rocks in Maryland to Rock Creek in the District.

Officials with the Potomac Conservancy nominated the site, considered by many to be the wildest, most natural river running through a major metropolitan area. The absence of strong riverfront protections, these officials maintain, has left the area under constant attack by developers.

In one case three years ago, a developer lacking permits cleared more than 60 trees in order to build a $100,000 staircase down to the river, a project that conservationists denounced as "a shining monument to arrogance." Fairfax County sued to get the trees replaced along the historic waterfront -- and the homeowner complied -- but environmentalists lamented that the long-term damage had been done.

Every year seems to bring new complaints about homeowners looking to maximize their view of the Potomac. Last summer in McLean, for example, construction began on a large gazebo visible from the American Legion Bridge.

"It sticks out there like a sore thumb," said Matthew Logan, president of the Potomac Conservancy, a regional land trust. "From a scenic standpoint, it's probably the most sensitive place you could build something. It completely alters the view.

"The problem is that this area is no longer just the natural Potomac gorge [from Great Falls to Key Bridge]. It's someone's back yard. . . . That's why we'd like to see it protected in all the ways we can."

Officials with Scenic America, which selected this year's top 10 from 50 nominations nationwide, agreed.

"If the Middle Potomac keeps going the way it is, with housing going in much too close to the water and cell towers going up and a series of actions happening independently without any sense of integrity of the corridor, you're going to have a mess," said Meg Maguire, president of the nonprofit organization, which seeks to preserve natural beauty and community character. "It won't be the quality, reflective experience it now is for boating, hiking, for being on that stretch of the Potomac."

Conservancy officials say solutions are within reach, starting with the adoption of much stronger ordinances governing the riverfront, requiring that new power plants be located in already developed industrial areas, and supporting transportation alternatives to a new bridge and outer beltway.

Also on Scenic America's list this year: the Blue Ridge Parkway in Roanoke County, Va.; the Massachusetts towns of Concord, Lexington, Lincoln and Bedford; the state Highway 99 corridor in California's San Joaquin Valley; and the Jordan River conservation corridor in Utah.

Scenic America released its first annual report in 1999, naming 12 Last Chance Landscapes. Among them was a historic swath from the Shenandoah to the Catoctin mountains, taking in parts of 10 Virginia counties, Harpers Ferry, W.Va., and Antietam National Battlefield in Sharpsburg, Md.

"We started this to highlight the different places where this inadvertent destruction of community character was occurring," Maguire said. "You could find examples in almost every community where things weren't done sensitively."

The goal, she said, is to get people talking.

Last year, she noted, after Scenic America selected some Georgia marshland where developers were building causeways to wetland islands and mansions, the state held hearings to discuss possible environmental damage to the area.

The group's 2001 effort to curb large wall signs in the District was not as successful. Preservationists had sought to stop their placement on downtown buildings, but the D.C. Council ruled that these "special signs" were exempt from the city's billboard restrictions. "They're trying to plaster the nation's capital with these things," Maguire said. "It was a very sad case of council cowardice. . . . We just gave away the store to the billboard industry."