Every weekend he can, Gene Elwell heads to the desert and races his buggy over the largest sand dunes in the U.S. Nearly 200 miles west, on California's Pacific shores, the Rev. John Fanestil spends every Sunday at Friendship Park, where people on both sides of the U.S.-Mexico border touch hands and talk through holes in a chain-link fence.

For decades, the dunes and Friendship Park were virtually unchanged. But in its final months, as the Bush administration raced to fulfill a pledge to erect 670 miles of fencing and vehicle barriers on the border, they were transformed almost overnight.

A fence now slices through the Imperial Sand Dunes, preventing recreational riders from veering into Mexican sands. Before, drug smugglers easily blended in with riders to reach Interstate 8, less than a half-mile from the border at one point.

Friendship Park, opened in 1971 to promote ties between the U.S. and Mexico, is closing. Crews have torn up a parking lot and removed trees and picnic benches to make way for two tall fences, ending years of cross-border food sales and family reunions.

It's similar elsewhere on the 1,954-mile border. In Eagle Pass, Texas, a golf course is sandwiched between the Rio Grande and a new fence. In Columbus, N.M., visitors see the fence from the high ground at Pancho Villa State Park, named for the Mexican general who led an attack there in 1916.

The Bush administration built 224 miles of barriers during its last 2 1/2 months, bringing the total to 602 miles. The Border Patrol plans to hit 670 miles this year, spokesman Lloyd Easterling said, but what happens after that is anyone's guess.

Homeland Security Secretary Janet Napolitano said at her Senate confirmation hearing that fences can help in border cities but that it makes little sense to fence the entire border.

Whatever happens, the border landscape already has indelibly changed.

The southern tip of the Imperial Sand Dunes Recreation Area, a film location for "Star Wars: Return of the Jedi," resembles the Sahara. Its slopes draw families because they are lower and more gradual than the desert playground's northern reaches, which attract rowdier crowds.

Elwell, 54, a San Diego-area native who sells office equipment, belongs to a close-knit but fast-growing group of families from Southern California and Arizona.

They ride the dunes day and night, sleeping in trailers parked around a campfire. They liken the thrill to a never-ending roller coaster.

"There's no particular trail, like you would up in the mountains or some places even out here in the desert," Elwell said. "Out here you find your own way, you find your own world."

Border Field State Park separates San Diego and Tijuana, Mexico, which together make up the largest metropolitan area along the border. It has a beautiful beach and abundant trails, but one of its biggest draws is Friendship Park, a half-acre cement plaza on a bluff overlooking the Pacific.

When then-first lady Pat Nixon opened the plaza 38 years ago, she declared, "I hate to see a fence anywhere."

On the beach, the countries are separated by steel poles spaced far enough apart at one point for an adult to squeeze through. Vendors in Mexico hawk corn-on-the-cob and tamales to customers on the U.S. side as Border Patrol agents keep watch.

Fanestil, 47, visited Friendship Park every Christmas to sing with revelers. The United Methodist minister was incensed in June when a Border Patrol agent said he no longer could pass food across the border. Fanestil responded by holding cross-border services every Sunday.

"People have been passing things through the fence here for generations _ tamales, candies, bracelets," the San Diego native said.

In California's southeast corner, the dunes are about 40 miles long and an average of five miles wide. Trailers crowd campgrounds on holiday weekends. The sand buggies range from lightweight speedsters with V-8 engines to lumbering vehicles that could pass for golf carts.

The sands extend about five miles into Mexico. Until last year, the border was almost invisible, marked by 15-foot concrete obelisks spaced far apart.

In the 1980s Elwell drove his buggy straight into the town of Algodones for tacos, and even in recent years he regularly drifted into Mexico. Agents chased wayward riders and ordered them back to the U.S.

Smugglers decorated their marijuana-laden vehicles with decals and flags to mix in. Last year, a suspected smuggler killed a Border Patrol agent by running over him in a Hummer, then fled to Mexico.

Border Patrol officials say private contractors were initially stumped when asked to design a 13-mile fence for the shifting sands. The answer was what the agency calls a "floating fence."

The $6 million-a-mile barrier completed in December consists of 16-foot-tall steel tubes filled with concrete and spaced tightly together. Triangular mounts aren't bolted to the ground, allowing them to rock back and forth with the wind. Small panels are chained together, twisting in different directions.

The Border Patrol says arrests of suspected smugglers plummeted after construction began last summer.

Elwell and his friends marveled as they stood on a ridge one recent Saturday and stared at the fence, stretching like a dark ribbon over the sand.

As the number of riders grows _ the dunes records about 1.2 million visits from October to March _ their playground is shrinking. In 2000, the U.S. Bureau of Land Management made about half the park off-limits to vehicles to protect the endangered Peirson's milk-vetch, a tiny perennial with purple blooms.

Some visitors feel the dunes are dangerously crowded, with up to 200,000 people some weekends. Steve Razo, a BLM spokesman, said seven to 10 people die each year in traffic accidents.

Elwell and his friends bemoan the tighter quarters but generally agree the fence will make the dunes safer.

"It makes you feel a little crammed in, like 'This is your playground, and you can't go over there,'" said Tom Holdenried, 59, a woodworker from Ramona.

At Friendship Park, the Border Patrol is building 3 1/2 miles of fence to the ocean, a project delayed for 12 years by lawsuits and environmental reviews. In 2005, the Bush administration exercised new authority to override those challenges; it later used the same powers on vast swaths of the border.

Work began in August and is expected to finish by May at a cost of about $16 million a mile.

The area is much quieter than it was in the 1990s, when the Border Patrol was badly overmatched by illegal immigrants who swarmed across the porous border. But Mike Fisher, the San Diego sector chief, said people still make holes in the fence and slip children between poles on the beach.

"I want people to be able to go to the park with their kids and not worry about smugglers operating in that area," he said.

Hilda Olivares drove from Los Angeles on one recent Sunday to talk through the fence with her mother, sister and brother-in-law. She could have seen them in Tijuana but wanted to avoid the long lines at the border. Her husband, Paul, passed them a wad of cash wrapped in a $20 bill.

Carlos Perez, who lives in Tijuana, cried when he saw his 5-year-old daughter, Caszandra, for the first time in three years. She slipped through the fence and embraced him, also in tears.

"A fence makes things more difficult," Perez said. "It divides us even further."

Added Sandra Castillo Vasquez, the girl's mother and a U.S. citizen who lives in San Diego: "At least we had this connection. We could touch each other."

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