Border between the US and Mexico is 2,000 miles long, spanning arid territory as well as urban center

If Republican presidential nominee Donald Trump is serious about his pledge to build a wall to stop the flow of illegal immigrants from Mexico to the United States, he has his work cut out for him.

US Border Patrol agents say they are aware of a new border crossing between Texas and Mexico that enables people to cross the Rio Grande during times of low tide.

According to KRGV, when the river drops to low tide, it is six-to-10 inches lower than normal, revealing a sand bar that makes it possible for would-be illegal aliens to cross the border.

The low tide lasts for days at a time, according to the report.

The Rio Grande, which forms part of the border between the United States and Mexico, is 1,900 miles long. A man is seen here walking along the sand bar during low tide

The 2,000-mile (3,200-kilometer) US-Mexico border runs from the Atlantic Ocean to the Pacific, crossing arid, sparsely populated territory as well as urban centers thick with inhabitants.

After initially promising to build a new barrier running the entire length of the border, Trump now says only half actually needs to be covered because the physical terrain acts as a natural barrier along the rest.

During a television appearance this week, the candidate was asked about the specifics of his planned barrier.

'I think the height could be 35 to 45 feet. That's a good height. That's a good height. Anywhere in that neighborhood. Could be higher,' he told Sean Hannity of Fox News earlier this week.

The low tide enables would-be illegal aliens to cross the border from Mexico to the United States through the Rio Grande, which runs along the boundary with Brownsville, Texas

The Republican nominee committed once again to building his proposed wall along the border with Mexico, to raucous cheers from the crowd.

'The Great Wall of Trump', as he has called it, has become a key component of his campaign.

'It's not a question of wanting [to build it]. We have no choice,' he said. 'It'll get done so quickly your head will spin.'

Trump's proposed wall height has changed several times during his campaign. In October Trump said that it could be 40-50 feet high.

US Border Patrol agents said that they are aware of the matter and are closely monitoring the area to prevent infiltrations

His cost estimations have also varied - on two separate occasions in February he put it at $8 billion and $10-12 billion.

It will be made of precast concrete, he says.

The number of immigrants illegally crossing over the Mexican border into America has risen sharply, according to newly released figures first reported this past May.

The US Customs and Border Protection (CBP) has revealed that child migration is on par with figures from 2014.

From October 2015 to March 2016, border officials stopped 27,754 unaccompanied children, representing a 78 percent jump from 15,616 apprehended in 2015, according to The Hill.com.

GOP presidential nominee Donald Trump has made building a barrier along the US-Mexico border a central plank of his election campaign

The issue of illegal immigration from Mexico has proven to be one of the most contentious issues in contemporary American politics.

Earlier this month, the mayor of a rustic Arizona town not far from the US-Mexico border angrily announced that he won’t be attending a bilateral conference of regional American and Mexican mayors because the invitation he received from the organizing body included English and Spanish translations.

Ken Taylor, the mayor of Huachuca City, rejected an invite from the US-Mexico Border Mayors Association because, in his words: ‘I will NOT attend a function that is sent to me in Spanish/Mexican.’

‘One nation means one language and I am insulted by the division caused by language,' Taylor wrote in an email to the BMA’s executive director, former El Paso, Texas mayor John Cook.

The BMA is an organization comprised of mayors from Mexican and American frontier towns and cities that is ‘recognized as a leading authority on border issues and will speak with a unified voice as we make recommendations to [legislatures and Congress] that will help the Mexico and the US border region grow and prosper economically.’

Taylor, however, wasn’t happy with the bilingual invite, according to the El Paso Times.

Cook replied to Taylor that he would no longer trouble him with correspondence from the association that sought to attract appeal from both sides of the border.

‘I will certainly remove you from our email list’, Cook wrote to Taylor. ‘Ours is a bi-national association with mayors from the United States and Mexico that were elected to serve border communities.’

'All of our communications are intended to inform mayors from both sides of our border about our association.’