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.