Scrapers and bulldozers began filling a deep canyon Friday to make way for a border fence in the southwestern corner of the United States after 12 years of planning, environmental reviews and legal challenges.

The 3 1/2-mile stretch extends from a state park on an oceanfront cliff through a canyon known as Smuggler's Gulch. The gorge was overrun by illegal immigrants until U.S. authorities launched a crackdown in the 1990s that pushed traffic to the remote mountains and deserts of California and Arizona.

At a cost of about $16 million a mile, the fence will be far more expensive than fences the U.S. government is building elsewhere along the nation's 1,952-mile border with Mexico. U.S. Customs and Border Protection said the average cost along the entire border is $2 million to $3 million a mile.

The stretch near San Diego will cost about $57 million under a contract awarded to Kiewit Corp. of Omaha, Neb., said James Swanson, a Border Patrol special operations supervisor.

The lion's share will pay for filling Smuggler's Gulch with nearly 1.9 million tons of dirt and for building a concrete culvert to handle rainfall flowing downhill from Tijuana, Mexico, Swanson said.

The border is currently marked by a decaying fence made of surplus Navy landing mats. Border Patrol agents swarm the area in jeeps and pickups as they wait for migrants in Tijuana to dash about 2 miles through trees to the closest patch of stores and homes.

It is a far cry from the early 1990s, when large groups blitzed across the border and easily overwhelmed the Border Patrol.

U.S. authorities insist new fencing is needed, despite an increase in patrols and objections from environmental groups who say the dirt shift threatens the Tijuana River estuary, home to more than 370 migratory and native birds.

"We're not seeing the thousands, the hundreds who streamed through in the past," said Mike Fisher, chief of the Border Patrol's San Diego sector. "However, it's still a vulnerability that's being exploited today."

Arrests along the stretch have doubled in the past year as the Border Patrol has added agents, said spokesman Alex Renteria. Arrests totaled 16,738 in the area from October through July, or about 60 a day, up from 7,944 the same period last year.

The project calls for a dirt access road and 15-foot steel mesh fence just north of the existing fence. Crews will also build a third fence about 10 feet high farther north and install lights.

"It's crazy," said Victor Clark Alfaro, director of the Binational Center for Human Rights in Tijuana. "I don't see the justification to spend $60 million on an area that's no longer an important crossing."

Clark predicted the new fence will reduce crossings to "almost zero" but inflict serious environmental damage.

The construction will help inch the Bush administration toward its pledge of 370 miles of pedestrian fencing and 300 miles of vehicle barriers on the U.S.-Mexico border by the end of this year. Homeland Security Secretary Michael Chertoff told a House committee last month that the government was "on track" to hit that mark, though it had built only 182 miles of pedestrian fence and 153 miles of vehicle barriers as of July 11.

Congress approved the fencing in a 14-mile stretch from the Pacific Ocean in 1996, but the government faced stiff opposition over the westernmost piece. In 2004, the California Coastal Commission refused to grant permits, saying damage to sensitive habitats outweighed security benefits.

In 2005, Chertoff overrode the commission's objections — as well as a federal lawsuit by the Sierra Club — by exercising new powers to waive legal and regulatory challenges to build the fence. He has since used that power to clear the way for hundreds of miles of fencing in California, Arizona, New Mexico and Texas.

According to local legend, Smuggler's Gulch got its name from alcohol that was smuggled into the U.S. during Prohibition. In Mexico, it is known as "Canon del Matadero" — or "Slaughterhouse Canyon" — supposedly because there was once a goat slaughterhouse nearby.