Popular or not, border experts say that Donald Trump’s border wall will be a logistical nightmare and physically impossible to build along the entire U.S.-Mexico border.

Currently, only about 700 miles of the entire 2,000-mile southern border has a fence. Construction on the current fence began under former President George W. Bush. UTSA border expert Omar Valerio-Jimenez, Ph.D., explained to KENS 5 why most of the border doesn’t have a physical barrier.

“Along the U.S.-Mexico border, there are areas where there is very rocky terrain: there are mountains, valleys,” Valerio-Jimenez said. “In Texas, there happens to be a lot of privately owned land.”

In Texas, there about 100 miles of fencing, but only near urban areas like El Paso and Brownsville. Valerio-Jimenez says that the Rio Grande River complicates construction.

“You can't build on a floodplain. For instance, in the Brownsville area, the wall has been built away from the river,” he said.

Estimates for a brand new wall along the southern border range from $10 billion to $25 billion. Both Republican and Democratic law makers from Texas say that the wall isn't the answer to our immigration problem.

In a statement, Republican Will Hurd, whose congressional district runs from San Antonio to El Paso, said that the wall doesn't belong in some parts of Texas:

"Big Bend National Park, and many areas in my district, are perfect examples of where a wall is unnecessary and would negatively impact the environment, private property rights, and economy. Building a wall is the most expensive and least effective way to secure the border.”

Democrat Joaquin Castro, who represents northwest San Antonio in Congress, called the wall a waste of taxpayer money.

“The border wall is a lazy and ineffective security strategy,” Congressman Castro said. “In fact, recent polls show that 59 percent of the American people disagree with building a wall along our nation’s southern border. The wall could cost up to $25 billion to construct – that’s American taxpayer money that could be used more effectively elsewhere, like in our schools or on crumbling roads.”