President Trump's aggressive bid to build a wall on the nation's southern border faces several hurdles on the ground, including endangered species, sacred tribal native territory and the likely use of eminent domain to take privately owned land, according to border experts.

But a wall built near Yuma, Ariz., proved so successful that illegal crossings were slashed 94 percent, according to Senate testimony previewed by Secrets and expected to be given today to the Senate Homeland Security and Governmental Affairs Committee.

Firmly stating that walls work and also help to preserve areas overrun with illegal immigration, the nation's former Border Patrol chief and former acting commissioner of U.S. Customs and Border Protection cited several issues that could entangle the Trump plan.

In prepared testimony, David Aguilar called on Congress to "waive all legal requirements," but noted several problems that could brake the wall. From his testimony:

There are numerous federally endangered or threatened species living along the border.

In Arizona, 85 percent of the land along the border are Federal lands set aside to protect wilderness and wildlife, such as Organ Pipe Cactus National Monument and the Cabeza Prieta National Wildlife Refuge.

Most of the land along the Texas border is privately owned. Landowners will have to be compensated for use of their lands for either construction or construction access. Eminent domain may have to be exercised to take land required for the construction of border infrastructure.

The Tohono O'odham nation occupies 75 miles of the U.S.-Mexico border in Arizona. The government will need to reach agreement with this native American nation to construct barriers on their land.

Nonetheless, he said a wall would help settle many of those issues. "It is important to note that there is nothing more destructive to environmentally sensitive land and quiet communities than the uncontrolled illegal flow of people, vehicles, smugglers, and criminal organizations. The placement of fences and deterrent infrastructure in previously uncontrolled parts of the border have actually allowed for the rejuvenation of areas that had previously been devastated due to heavy illegal pedestrian and vehicular traffic," he said.

In his opening statement, committee Chairman Sen. Ron Johnson said, "The purpose of these additional border barriers is to gain control over the southwest border. As [Homeland Security] Secretary Kelly testified to the committee in January, 'the number one threat to the nation is that we do not have control of our borders. Without control, every other kind of threat—drugs, illegal migrants, counterfeit manufactured goods and pharmaceuticals, diseases, terrorists, and the list goes on—can enter at will, and does."

Also testifying is Ronald S. Colburn, the former deputy chief of the Border Patrol. He said border walls are effective. For proof, he cited construction of one at Yuma, Ariz., that stopped illegal crossings cold. "By 2008, Yuma Sector arrests of illicit border crossers and traffickers had dwindled from over 138,000 down to 8,363. The known attempts to enter and the got-aways dwindled to an equally minimal number compared to the hundreds of thousands that entered and evaded arrest in previous years," he said.

Colburn provided a before and after example of progress:

— Before fence: Yuma Border Patrol recorded 2,706 known "drive-throughs" in a one year period. This is where smugglers load up vehicles with their contraband of drugs and people, and simply drive across the open, unfettered border, and cross the river in shallow places, destroying wilderness landscape along the way. They lose themselves in urban areas and traffic once reaching paved roads. Of the 2,706 drive-throughs, we recorded a mere 13 captures and turn backs. The rest all got away, with no idea what or who they brought in.

— After fence: Only six vehicles attempted to enter, at other than a designated port of entry. None got away – we captured or turned back all of them. From 2,706, down to six. Impressive results.

Paul Bedard, the Washington Examiner's "Washington Secrets" columnist, can be contacted at pbedard@washingtonexaminer.com