CALEXICO, Calif. — A wall separates the United States from the Mexican metropolis of Mexicali in this city 120 miles east of San Diego.

Two stories tall, it runs west for dozens of miles, losing its thick steel mesh and razor wire to become a waist-high barrier before abruptly ending in the Yuha Desert.

Congress ordered the construction of 700 miles of fence line in 2006; the Border Patrol continues to maintain it, with crews constantly detailed to “ride the fence and repair it,” Michael J. Fisher, the

agency's chief, told the Tribune-Review.

During almost a week that Trib journalists spent in Calexico, it was patrolled constantly by Border Patrol trucks, some parked feet from one another, with special canopies to ward off rocks

thrown by smugglers as a distraction while illegal immigrants scurry over the fences.Although the wall cost American taxpayers about $3 million per mile, border jumpers can get over it in seconds

They poke cheap screwdrivers into the metal mesh and use them as pegs to climb to the top of the fence. Then they slide down ropes and sprint to American smugglers, or “coyotes,” who ferry them north into American cities

and towns.

That's why frayed nylon line and gloves, worn to prevent friction burns, litter the base of the wall.

“People say, ‘Build a fence; build a wall.' But that really is only good at stopping some vehicle drive-throughs. Smugglers will saw through the fence or go around it. It's not really designed to stop people.

It's like a speed bump. It just slows people down,” said Shawn Moran, a Border Patrol agent with two decades of experience.

Moran is vice president of the National Border Patrol Council, the union representing about 21,300 federal agents, most of them stationed along the border with Mexico.

Texas Gov. Tom Abbott, a Republican, recently called for 250 Border Patrol agents to be assigned to his state, as a result of reports that the Department of Homeland Security detained almost 10,000 immigrant families and

unaccompanied children in August. Those detentions were 50 percent higher than in August 2014, but they buck the larger, overall trend.

Through Aug. 31, Border Patrol's most recent report, agents detained 35,494 unescorted youngsters this federal fiscal year — down 46 percent from the same time last year. During that same span, Homeland Security apprehended

34,565 illegal immigrants traveling together as families — a drop of 48 percent compared to the same time last year.

Homeland Security officials reported a total of 151,805 apprehensions of undocumented immigrants across the southwest border region in their most recent available report in April, the lowest total in four federal fiscal years.

Through March 31, apprehensions were down 28 percent compared to the same time in 2014.

New York billionaire Donald Trump, the front-runner for the Republican presidential nomination, has made a sturdy border wall the cornerstone of his campaign.

“It'll be a wall that works. It'll actually be a wall that will look good, believe it or not. 'Cause what they have now is a joke. ... They're ugly, little and don't work,” Trump said recently on CBS' “60 Minutes.”

A CNN poll released Sept. 14 revealed that more than half of Americans support Trump's call for a bigger, better wall.

Similar barrier-building promises are echoed by GOP hopefuls Ted Cruz, Mike Huckabee and John Kasich, but Republican candidates Jeb Bush, Ben Carson, Carly Fiorina,

Marco Rubio and Chris Christie have urged stiffer security without endorsing more fences. That position is shared by most Democratic presidential candidates and the federal agencies that police the border.

During a two-month journey along the 1,954 miles of the U.S.-Mexico border conducted as part of its “American Coyote” series published in July, the Trib found little evidence to suggest that more or bigger walls would prevent incursions.

Walls are popular with ranchers weary of illegal immigrants trespassing on their land. But mayors, federal agents, law enforcement officials, members of the Texas National Guard and coyotes told the Trib that a large wall isn't a cure

for complex smuggling operations used by 90 percent of illegal border crossers.

In December, for example, the Trib tracked a group of undocumented Guatemalan immigrants who shimmied over a low fence in Arizona's Coronado National Forest. Before Border Patrol agents chased them back into Mexico,

they were hiking toward Miller Peak, a summit 5,000 feet above the fence line — far higher than the tallest of walls.

Trump's border wall proposal is unpopular in Mexico City, where officials fret that it will hurt “bilateral relations and obstruct effective collaboration between neighboring countries and partners,” said Tania Rión, spokeswoman for

the Mexican Embassy in Washington.

What's most confusing to Rión is that the push for bigger fortifications occurs during an era of increasingly robust border security and plummeting numbers of undocumented people attempting to cross into the United States.

Homeland Security reports detentions of undocumented immigrants have dropped 28 percent compared to the same time last year. Averaging about 25,300 unauthorized crossers per month in the first half of the fiscal year, officials

pointed out that illegal immigration has fallen to levels unseen since the 1970s.

Voters pushing for a border wall, however, seem to believe the opposite.

Fisher said “every data, every piece of information” shows that is not the case.

“There's a huge perception problem, and that's one of our challenges,” the Border Patrol chief said.

Resulting from the Sept. 11, 2001, terror attacks, Congress doubled the size of the Border Patrol to the point at which there are the equivalent of about nine agents for every mile of border with Mexico, although all are not stationed there.

By early 2013, federal officials realized that the growth in agents didn't always flow to where they were needed most. Technology, from drone aircraft to surveillance sensors, increasingly showed vast expanses of the southwestern

states where coyotes didn't operate, largely because the terrain makes human trafficking difficult.

A Trib analysis of 3,254 coyotes convicted in federal courts along the Southwest border in 2013 and 2014 found that prices often followed policing: The stiffer the border security, the more it cost undocumented immigrants to get into

the United States.

A one-way ticket from a Mexico border city to San Diego or Los Angeles, for example, ran about $6,784, more than twice the fee to get smuggled from Mexico up through the southern toe of Texas to Houston, the busiest

trafficking corridor in America.

Budget woes have largely prevented Border Patrol from growing the size of its force or even paying the costs to shift agents permanently to places where analysis has since determined they're more chronically needed, such as

Arizona and south Texas.

As part of the so-called sequestration deal to stave off a federal budget impasse in 2013, Congress ordered Customs and Border Protection to cut $754 million spent on hiring, travel, training, facilities and gear.

“Border Patrol's budget is approximately between $3.4 billion and $3.5 billion. And 92 percent of that goes to pay salaries and expenses. So when you're mandated by Congress to maintain a certain staffing level, and your budgets don't

rise every year as your workforce gets more expensive, we continually have less and less operational money,” Fisher said.

Moran proposes a fix: Hire 5,000 agents and let some of them push north into major cities such as Phoenix, bureaucratic turf controlled by Immigration and Customs Enforcement, a sister Homeland Security agency that's tasked

with deporting undocumented aliens from the nation's interior.

He believes that adding checkpoints and patrols inland would increase arrests; operations within 100 miles of the border account for about one of every four arrests, records show.

Congressional researchers peg the costs of the union's proposed hiring initiative at about $857 million per year. The White House has asked to hike spending for the Customs and Border Protection by $803 million — far less than

the new hires would cost. But Moran believes that the mood in Congress and across the country is to spend more.

“For too long, we've been trying to do border security on the cheap,” he said. “Now there's political will for agents to enforce the law.”

.Read more: Border wall slows illegal crossers from Mexico but can't stop them | TribLIVE