ORGAN PIPE CACTUS NATIONAL MONUMENT — While politicians in Phoenix and Congress talk about building a tall fence along Arizona's border with Mexico, workers here are completing a shorter and more modest obstacle.

This low-slung vehicle barrier will do nothing to stop people from walking into Southern Arizona illegally.

But on public lands where the obstacles are popping up, officials say the devices have succeeded in stopping the so-called drive-throughs that can imperil law enforcement and scar the thin-skinned desert for decades.

Homeland Security and other officials have disclosed plans to build similar barriers along most of the border between Yuma and Nogales, though the cost and timing of many proposals are unclear. On Thursday, the Senate Judiciary Committee approved at least 200 miles of barriers along Arizona's border with Mexico.

To some, barriers that block only vehicles are Band-Aids on a hemorrhaging wound. Vehicles can still drive around the ends, and smugglers are already scheming ways to defeat the new obstacles.

In a 2004 report by the House Select Committee on Homeland Security, congressional staff reported seeing photos of a special truck that could park perpendicular to the barrier on the Mexican side, unfold a set of rails over the barrier and unfold another set of rails behind it.

"Other vehicles could then drive up the back of the modified truck, across its top and down its front, over the vehicle barrier and into the United States," the report said.

To build the barriers, workers may need to widen roads, build new ones and sink wells to supply water for making concrete. Despite those impacts, biologists, land managers and environmentalists generally favor the barriers because they don't impede wildlife and can prevent even greater damage by vehicles.

It's possible, however, that the same roads built in support of the vehicle barrier could later be used to construct far more elaborate infrastructure, including the fence the House of Representatives approved in December for nearly all of the Arizona-Mexico border.

Scientists say such a fence would fragment habitat and block movements of many species that ignore the international boundary. Although most birds and bats could bypass the fence, creatures varying from tortoises to coyotes to endangered jaguars could not.

Jaguars, commonly associated with the tropics, were historical residents of the Southwest and have been filmed repeatedly in Southern Arizona over the past decade. There was another confirmed sighting in the New Mexico Boot Heel last month, presumably a cat that had dispersed from a breeding colony 150 miles south of Douglas.

Whatever environmental impacts a big fence might cause could be moot because last year Congress gave the Department of Homeland Security the ability to exempt itself from laws like the Endangered Species Act.

The damage also must be weighed against the status quo of a porous border, which everyone agrees is harming Southern Arizona's plants and animals.

"It's both a human tragedy and an ecological tragedy," said Lisa Haynes, a University of Arizona wildlife biologist.

Vehicle incursions common

In a sign of how permeable the border is, thousands of vehicles enter Arizona illegally each year, ferrying drugs and people through protected areas where driving is prohibited.

In 2003, Tohono O'odham police reported towing 7,000 stolen vehicles from their reservation. More than 2,700 vehicles crossed illegally through the Border Patrol's Yuma Sector in fiscal year 2005.

At Coronado National Memorial, which borders Mexico for 3.5 miles south of Sierra Vista, nearly a mile of barrier "has changed the whole pattern of vehicle travel in this area tremendously," superintendent Kym Hall said.

On the Mexican side, roads have been bulldozed right up to the border of the memorial. Before the erection of the barrier — which will be extended by a mile this year — there were several vehicle incursions each week. Last summer, rangers noticed a truck speeding out of an unpopulated canyon with a refrigerator in its bed — possibly holding drugs — and were nearly rammed before firing at it.

"It nearly killed my chief ranger," Hall said.

At Organ Pipe Cactus National Monument, south of Ajo , officials pleaded for a vehicle obstacle for years. But funding wasn't secured until ranger Kris Eggle was murdered in 2002. Eggle, 28, was pursuing a drug-smuggling suspect who drove through the flimsy barbed-wire fence just east of Lukeville. Eggle's killer was later shot to death by Mexican agents firing across the border.

In the wake of Eggle's death, the National Park Service approved an $18 million, 30-mile vehicle barrier along the park's entire border with Mexico. Construction has already taken more than two years — surveys for the endangered pygmy owl caused some delays — and park officials hope the project will be complete this summer.

"A spider web of trails"

The most common style of barrier at the park features a horizontal rail three feet above the ground and welded to vertical rails anchored in a five-foot hole with concrete. Where concrete-filled tubes are used as posts, engineers say the barrier can withstand the impact of a 7,000-pound vehicle hitting it at 40 mph and take an hour to cut through.

In extremely rugged areas, "Normandy barriers" similar to what troops encountered on the French beaches on D-Day are dropped by helicopters and kept in place by weight.

Smugglers have breached the Organ Pipe barrier once, but park officials say drive-throughs have been virtually eliminated where the obstacle is installed.

Walkers, however, easily jump over or slide under the Organ Pipe barrier. At peak times, 1,000 crossers still hike through the park each day, trampling cacti, compacting soil and accelerating erosion. The park issues fewer than 100 backcountry permits each year and has been forced to close nearly all of its main loop road indefinitely due to the illegal traffic.

"Just by walking they're creating a spider web of trails and there's trash all along those trails," superintendent Kathy Billings said.

Dripping Springs, a rare water source in the area, has been contaminated by E. coli bacteria, presumably from the waste of border crossers.

"You walk through some of the passes and the smell overwhelms you," Billings said.

A dozen miles north of Dripping Springs, a Border Patrol agent caught up to a group of illegal entrants in the park late last month. Carlos Jimenez, a 32-year-old from the Mexican state of Zacatecas, had ridden 22 hours in a bus, then jumped the vehicle barrier near Lukeville.

"They told us it would be a six-hour walk to Phoenix," he said.

It had already taken the group a day to walk 20 miles through Organ Pipe. A straight line from where Jimenez was captured to Phoenix's Sky Harbor Airport extends another 105 miles. When the group's journey was complete, their guide —who eluded capture — would get paid $1,600, Jimenez said.

At home, Jimenez said he could earn $10 a day. In Phoenix he expected to make $8 an hour in construction or landscaping.

"We're just trying to support our families," he said.

Billions for big fence

Hoping to keep people like Jimenez out of the country, the House of Representatives last year approved 698 miles of fencing across the U.S.-Mexican border. Two layers of fences 10 to 15 feet tall would be separated by 50 yards and a road that agents could patrol. Lights, sensors and cameras would assist in monitoring the fence.

Rep. Duncan Hunter R-Calif., author of the fence amendment, says a similar barrier in his San Diego district has kept people out — and let hammered vegetation recover. Hunter's office estimates the fence would cost $2.2 billion, or $3.15 million per mile.

"Given what illegal immigration costs each year, given that 30 percent of inmates in our federal prisons are illegal aliens and given the strain illegal immigration puts on our health care, it's a small price to pay," Hunter aide Joe Kasper said.

Fencing can be effective in urban areas, but in more desolate areas "we don't have the infrastructure or technology to back it up," said Gustavo Soto, spokesman for the Border Patrol's Tucson Sector.

About five miles north of where the fence would be built, visitors to Papago Well in Cabeza Prieta National Wildlife Refuge offered their opinions on the idea.

"There would just be more tunnels," said Joseph Gordon, 48, one of three registered nurses from Tucson camping along the fabled Camino del Diablo.

George Mortimer, 57, said the government could really stop the problem of illegal immigration overnight.

"Simply make hiring someone without a passport an act of sedition," he said. "But everyone benefits from it, so there's no political will to stop it."

Nearby, a group of Yuma-area retirees had a more favorable impression of the fence.

Buz Gellenbeck, 69, a retired painting and drywall contractor, said he'd support a fence "if that's what it takes" to stop illegal border crossers.

"It'd be better than having something like what happened on 9/11," he said.

Wilderness hammered

On the Camino del Diablo and other routes in the refuge it's not uncommon to find swaths of Sonoran Desert converted to "moon dust" the consistency of talcum powder. Even the biggest pickups and SUVs get stuck, so agents and smugglers drive around the pits and make the road several hundred yards wide in spots.

Abandoned vehicles litter federal wilderness, where it's typically illegal to drive, but towing them out can cause even more damage and using helicopters can cost $3,000 an hour.

A $24 million vehicle barrier similar to the one in Organ Pipe has been proposed along 38 of the 56 miles of border in Cabeza Prieta. But the Border Patrol is considering using a different design in which a piece of heavy equipment on tracks could sink poles in the ground four feet apart. That would minimize the need for new roads and wells.

Cabeza Prieta currently has two law enforcement officers to patrol its 860,000 acres, though two more are in training. One of the officers, Jeremy Bucher, said a Border Patrol encampment in the middle of the refuge has cut down on drive-throughs.

"You could have hundreds of agents here on this 56 miles of border on the refuge and there would still be some sizable gaps," he said.

Bucher, who was an agent in the Border Patrol's Ajo station from 1998 and 2003, estimated 10 percent to 20 percent of illegal crossers in the area are caught.

Waivers for border security

Right along the border, a 1907 proclamation signed by President Theodore Roosevelt reserves a 60-foot wide strip of land for patrols and protective measures. Farther inland, Homeland Security could bypass all environmental laws, including those protecting wilderness and endangered species.

A provision in the Real ID Act, passed in March 2005, gave the agency "authority to waive all legal requirements" in order to get a 14-mile fence built near San Diego that was stalled for years over ecological concerns. Wielding the power in September, Homeland Security Secretary Michael Chertoff said, "I reserve the authority to make further waivers from time to time."

David BeMiller, the Border Patrol's first public-lands liaison, said environmental regulations "add a couple of steps to the process for us to take enforcement action, but we recognize we're obligated to adhere to all federal laws."

Defenders of Wildlife describes the waiver as "a breathtaking transfer of power."

"What little public input is allowed now would be completely shut off," said Jenny Neeley, Southwest representative for the environmental group.

Public-land managers in Southern Arizona said they aren't expecting the Real ID Act exemption to be exercised here. But Cabeza Prieta manager Roger DiRosa likens the waiver to "the sword of Damocles."

"If terrorists blow up the federal building in Phoenix and they trace them to going through the Cabeza Prieta," he said, "then all bets are off."

Border bills to beef up security

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● Some of the proposals for beefing up border security:

l On Thursday, the Senate Judiciary Committee approved a plan to add up to 50 miles of fencing and 200 miles of vehicle barriers along the Arizona-Mexico border. The measure would also double the number of Border Patrol agents in the nation from about 11,000 to more than 21,000 over the next five years.

l In December, the House of Representatives approved HR 4437, which calls for 698 miles of fencing along the border. The largest segment would stretch 361 miles from Calexico, Calif. to just east of Douglas. The bill is awaiting action in the Senate.

l In the state Legislature, HCR 2037 would support creation of a border fence with an 8 percent tax on all electronic transfers of money between Arizona and foreign countries, except for goods and services.

l Arizona HB 2003 would give Yuma, Pima, Santa Cruz and Cochise counties $6 million to construct a dirt road along the border.