For more than a decade, the "Fenway trails" were the center of the local BMX scene. Lovingly maintained and legally questionable, the series of dirt jumps were built on an unused parcel of MBTA land next to the Fenway stop on the Green Line. On April 25, a backhoe arrived and leveled the mounds in a couple of hours.

Sure, the BMX riders would have liked a little notice, but they had always known the day would come. "When a dirt park is on private land, it's on borrowed time," says Jody Stoddard, owner of Timeless BMX in Cambridge and a regular at the Fenway trails.

Now Stoddard and the rest of the local BMX scene are looking for a place to go. Some think they have found the perfect spot: amidst the tangle of on and offramps and concrete factories underneath the Leonard P. Zakim Bridge, where the fans of bicycle moto-cross could race on earthen tracks.

The BMX riders are hoping to piggyback on the plans for the Charles River Skatepark, the $2.3 million, 40,000-suare-foot facility for skateboarders, freestyle bicyclists, and inline skaters that is being proposed for the west side of the bridge, shepherded by the Charles River Conservancy. But while the skatepark is a massive project—it will take at least a year to pour the hand-troweled, finished concrete—and is still only halfway to its fund-raising goal, Stoddard says the BMX trails would cost next to nothing and could be up and running in no time.

"Six weeks, some water, a couple shovels, and a bunch of dirt," Stoddard said as he looked out over the potential site, a triangular plot dotted with support columns directly below the Zakim Bridge on the north side of the Charles River. "That's all it would take. And we can probably get the dirt donated."

The idea for the trails came about last fall, when Renata von Tscharner, president of the Charles River Conservancy, approached Stoddard with the idea of including a BMX trail amid the parklands planned for a stretch of riverfront that has been a sore sight for decades.

"I pointed out the location to him because it's not really used for anything," von Tscharner said of the land, which is controlled by the Massachusetts Turnpike Authority and would be connected to the skatepark via a pedestrian bridge that is already a part of the parkland proposal. "It's just a bunch of gravel underneath the highway ramps, but it ties in with the notion of bringing young kids to the river, giving them a place to show off, and making the parklands more active and accessible."

But with the demise of the Fenway trails, the Charles River dirt park proposal—which von Tscharner describes as being in the "very early stages"—has taken on new urgency as BMX riders look to their first summer without jumps in more than a decade. Within two weeks of the demolition of the Fenway trails, more than 400 people signed an online petition in support of the Charles River proposal.

As of last weekend, when skatepark supporters met at the main branch of the Boston Public Library for a presentation by the park's designer, the capital campaign for the project had received $475,000 in grants from foundations, nearly that much from private individuals, and $300,000 in public funding from the state, the Boston Redevelopment Authority, and the city of Cambridge. If all goes as planned, von Tscharner said, she hopes to break ground on the skatepark as early as next year.

At 40,000 square feet, the Charles River Skatepark would be among the largest of the 800 in the United States, according to Zachary Wormhoudt, the California-based architect and lifelong skater who designed the plans after receiving input from dozens of local skaters and bicyclists.

As he walked the skatepark site recently, shaded by the tangle of swooping concrete with the massive mounds of the Boston Sand & Gravel Co. off in the distance, Wormhoudt pointed out the features that will someday dominate the park, like the 22-foot wave-shaped full pipe that will be its signature element "if you don't count the highway ramps overhead," he said.

He paused to let a Duck Tour pass on its way to splash into the Charles a few feet down the road, and took one final look at the spot before heading for the airport.

"This is a special place," he said. "To picture people down here skating and riding with the city as a background, and all the energy down here, it's going to be rad."