It's known as the water-balloon effect: Squeeze one spot and illegal immigration will bulge elsewhere along the 1,952-mile frontier.
While overall arrests have fallen a modest 3 percent since October, they are up sharply in some places, including the San Diego area. Thanks to a surge in hiring new agents, the Border Patrol says it's ready for a shift in traffic. Skeptics aren't so sure.
In the early 1990s, San Diego was overrun by border crossers. Hundreds at a time stormed the world's busiest border crossing, paralyzing motorists on Interstate 5. Migrants waltzed freely over to vendors who catered to them in nearby canyons.
A crackdown launched in 1994 and modeled on a similar effort in El Paso, Texas, pushed many migrants away from the border's two largest cities and into Arizona's mountains and deserts. Total arrests ebbed and flowed over the last decade but changed little: 1.3 million in 1995 versus 1.2 million in 2005.
Arrests in the Border Patrol's sector around Tucson, Ariz., are down 9 percent to 345,973 since October compared to the previous year, though it is still the busiest corridor. Meanwhile, arrests rose 19 percent to 175,324 between the two sectors that span all but a few miles of California's border with Mexico.
The recent arrest spike in and around San Diego comes as the Border Patrol grows from 11,800 agents today to 18,000 by the end of 2008. That force will be supported by up to 6,000 National Guard troops.
Border Patrol Chief David Aguilar said last month that San Diego is "very well prepared."
Skeptics say that no matter how many agents there are or where they are positioned, the rush of border crossers would continue as long as jobs were easy to get.
"You can put a million agents along the U.S.-Mexican border and that alone is not going to stop the pressure and flow of migrants," said David Shirk, director of the University of San Diego's Trans-Border Institute.
The spike in traffic is invisible to most San Diego residents because migrants cross in rugged terrain east of the seaside city of 1.3 million people.
The mountains and canyons are scorching in summer and freezing in winter. Unlike Arizona, tall trees and dense chaparral offer shade and hiding places. It is not an easy place to patrol.
At midnight one muggy July night, Nicholas Coates and other Border Patrol agents converged on an area where sensors suggested a group of about 10 migrants was walking. He tracked fresh footprints and broken twigs on a dirt path, then wrestled thick brush.
"They could be right here," he said around 2:30 a.m., shortly before giving up. "If they get in the brush, it's very hard to see them."
As an early sun burned, he captured three migrants in another large group after a two-hour chase, mostly on foot and with an assist from a helicopter and other agents.
The mountains surround California's Tecate, about 35 miles east of San Diego, which is home to little more than a small strip mall.
Mexico's Tecate is a pleasant town of about 60,000 people with a shaded central square, a spa that draws American tourists and a hulking brewery that bears its name. It doesn't look like a pit stop for border crossers.
A migrant shelter will grow from 20 beds to 200 when it moves to a new building in a few months, but the bus station is tiny and quiet, and there are few boarding houses. Unlike other border towns, few men with backpacks mill around.
Border crossers hop off a bus or taxi somewhere outside the city on a short trip from Tijuana, a city of 1.2 million people whose airport and bus station teem with smugglers looking for business.
Many migrants rest at Tijuana's red-light district hotels before the trek, which lasts at least two days. Drivers wait for them on the U.S. side to take them to a safe house.
"People come here when they tried elsewhere and failed," said Daniel Rivera, 63, who works for smugglers looking for customers near the 'Aqui Te Espero' ('I'll Wait for You Here') cafe in Tijuana's red-light district.
He said it costs $1,600 to be guided through the mountains near Tecate, typically in groups of four or five men, up from about $300 in the early 1990s. That inflation reflects how much harder it has become since the U.S. government tightened the noose in urban areas.
Word that hundreds of migrants have died crossing Arizona's unforgiving deserts may also contribute to the shift to California, said Wayne Cornelius, a political scientist at the University of California, San Diego. His interviews this year with 724 people in the Mexican state of Yucatan found San Diego was by far the favored crossing area.
Jose Reyes, 32, had enough of Arizona after he and 16 others were picked up by the Border Patrol three months ago. After crossing near Douglas, they walked five days and drank water from cattle tanks.
"We turned ourselves in (to the Border Patrol); they were our salvation," said Reyes, who spoke at a Tijuana migrant shelter. He planned to attempt another crossing, this time near Tecate, on his way to a dishwasher job in San Francisco.