A craggy border canyon in San Diego is difficult to monitor and too rough to handle much of a fence, so the federal government has come up with an idea — it will fill the 230-foot-deep chasm and get the dirt to do it by lopping off the tops of two nearby mesas.

By smoothing out Smuggler's Gulch with enough dirt to fill 70,000 dump trucks, the U.S. Border Patrol expects to be able to better patrol one of San Diego's weak spots.

Without major changes like the ones the Smuggler's Gulch plan calls for, much of the U.S.-Mexican border cannot be fenced, a Star investigation found.

Of the border's nearly 2,000 miles, 85 — 4 percent — are fenced. Another 61 miles have shorter barriers that keep cars from passing but let people and wildlife through.

Though rough terrain hinders agents' ability to patrol, reinforcing security with walls in rugged areas actually could help smugglers by providing an infrastructure — walls require roads to patrol them. Border fencing also would cost $2 billion to $5 billion or more and worries environmentalists concerned with water, wilderness preservation and animals.

"Parts of the lower Rio Grande are pretty wild. There are mountains that are really wild. There's the cost of equipment to consider," says Douglas S. Massey, a professor of sociology and public affairs at Princeton University who has studied international migration for nearly three decades.

"If you want to throw enough money at it, yes, you could do it," he says. "But you'd need to fill in 1,000-foot canyons, grade mountains. And a wall wouldn't be any good if you didn't have someone to patrol it."