Matt York/AP Photo

Standing off the rim of Kimmer Wash, just east of the border town of Nogales, Arizona, I watched as Border Patrol agents questioned the five men after making arrests this Wednesday. The men were caught about one half-mile into the United States after they climbed over the border fence. For these agents, this was another day’s work under the scorching hot sun along the U.S.-Mexico border.

Amid the influx crossing the border, some politicians are renewing their calls to extend the fence, like the one here in Nogales, across the entire U.S.-Mexico border. But the reality is that many Border Patrol agents, like those I spent time with, only find a physical barrier useful in stopping illegal crossings in urban areas like Nogales.

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In more rural parts of the border, like many parts of the Rio Grande Valley where the most recent wave of migrants have crossed, a fence is of little value.

The fence is an imposing structure. It can range from eighteen to thirty feet tall, depending on the location. The latest fencing is designed with gaps between each grate so that agents can peer through it.

But the fence is not a cure-all. “No matter how tall you build it, they will find a way over,” one agent said. (The agents quoted in here each spoke with me on the condition of anonymity.) While some Republicans have called for a more lethal barricade; one that has barbed wire or one that is electrified, the two agents I was driving with instead focused on how many injuries already happen at the fence due to falls. The Border Patrol has a unit dedicated to saving lives of immigrants. Even without electricity or barbed wire, the fence is a “constant source of injuries,” a University of Arizona doctor who treats the migrants said last year.

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The migrants we came across this week jumped over the fence and then stayed low in brush to avoid detection. The Border Patrol had a camera in the area that observed them and agents rushed to the area to make the arrest.

Rep. Mike Rogers (R-Ala.), right, is one lawmaker calling for more border fencing in the Rio Grande Valley (Susan Walsh/AP Photo).

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For some, it may have been their first time trying to cross the border. For others, it could have been just the latest of many attempts. One agent recalled arresting a man who, as his fingerprint records showed, was on his 103rd attempt to cross the border. If they had no prior convictions, the men would likely be processed quickly and sent back to Mexico that evening.

The fence is supposed to “slow down” the crossers, the agent said. Border-crossers will oftentimes, as the agent explained, use a ladder to get across and then descend a rope on the other side. They may leave the rope attached on the American side so they can retreat back over the fence if Border Patrol spots them. Others will simply “throw some dust on their hands,” and climb the fence, another agent said.

On the border, intelligence is the most sought-after commodity. The cartels keep close tabs on agents and try to find when they are not patrolling a particular part of the fence, or even when cameras are turned the other direction. Just across the border in Mexico, an infamous tan "lookout" house with windows on all sides is well-known by Border Patrol agents to be a cartel observation post. When agents patrol the rims, they find spots where scouts camped—usually marked by piles of empty tuna cans. During June and July, an agent said, there was a huge drop in smuggling because scouts took time off for the World Cup.

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Rather than building more fencing, the Border Patrol is deploying more surveillance technology themselves. In the deep valleys that lie west of Nogales, the Border Patrol is installing a new Integrated Fixed Towers system, a network of cameras that will provide new intelligence about movement in the most heavily used smuggling routes.

There's evidence that these new technologies and influx of manpower at the border are working. Apprehensions have been at record low levels for the past three years, according to government data.

On the American side, agents in a Nogales control room receive information from sensors and video feeds along the border. They try to spot people who jump across the fence and to detect tunnels that can go under it.

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In more rural sections of the border, where the terrain is marked by rough hills, agents have hours to respond to crossers before they reach a city. In those areas, a fence is of little use; the terrain slows down smugglers who often lug packs of marijuana that weigh as much as 70 pounds. To catch these smugglers, agents need intelligence, not a fence that can be climbed, cut or dug under.

The talk of "complete the danged fence,” as John McCain said during his 2010 campaign, may make for good politics, but the reality is that the fence is not a useful tool in the rural stretches of the border.

Samuel Kleiner, a third-generation Tucsonan, is a fellow at the Yale Law and Information Society Project.