The Mariscal Mountain road plan being pursued by U.S. Customs and Border Protection (CBP) inside Big Bend National Park could violate a binding international boundary treaty with Mexico and destroy what is described as the most densely populated peregrine falcon nesting area in Texas, according to a congressional letter and statements from environmental groups.
CBP, the largest federal law enforcement agency within the Department of Homeland Security (DHS), recently updated its Smart Wall map to include a proposed route passing over Mariscal Mountain, a remote section of the park in southwest Texas. The update represents the latest revision to a project that has undergone repeated changes since the Trump administration announced plans in early 2026 to build a border wall inside the protected area. The initially proposed wall was replaced in mid-June with a package including 17 miles of metal barriers and 205 miles of surveillance infrastructure, with a budget of $1.7 billion and a completion deadline of 11 May 2027. Contractor Southwest Valley Constructors Co. is named on the project.
Congressional Letter Raises Treaty Concerns Over Mariscal Mountain Road Plan
Texas Congressman Lloyd Doggett, together with 11 other members of Congress, has written to Secretary of State Marco Rubio, Homeland Security Secretary Markwayne Mullin, CBP Commissioner Rodney S. Scott, and W.C. ‘Chad’ McIntosh, Commissioner of the U.S. section of the International Boundary and Water Commission (IBWC), urging a reconsideration of the route.
Doggett described the project as an ‘incredible waste of both taxpayer money and an invaluable place of spectacular beauty.’ In the letter, he warned that ‘blasting a road across the mountain could send spoil toward, and potentially into, the Rio Grande,’ adding that ‘tons of rock blasted away could end up impacting Mariscal Canyon, the Rio Grande, and Mexico.’
The concern centres on Article IV of the 1970 Boundary Treaty, which requires the IBWC to review plans that could alter the river’s natural flow before construction proceeds. The treaty’s obligations run further still. According to EveryCRSReport.com, the treaty prohibits the United States and Mexico from constructing works that would deflect or obstruct the normal flow of the rivers, and requires each nation to remove prohibited works and pay damages where the IBWC determines those works have caused adverse effects. Should blast debris reach the Rio Grande and alter its flow, that liability framework could be engaged.
At present, CBP has not released engineering or design details for the proposed road. The only information made public concerns its location and a proposed width of 24 feet. Congressman Doggett’s letter requests spoil disposal plans and full design and engineering documentation.
Environmental Groups Question the Security Case for the Route
Bob Krumenaker, chair of Keep Big Bend Wild and former Superintendent of Big Bend National Park, told TheTravel in an emailed statement that the proposed road serves no meaningful security function. He pointed to the absence of any Mexican road access on the south side of the river at that location, making vehicle crossings there a practical impossibility.
‘The planned patrol road over the mountain is not just highly destructive, but it serves no security purpose, as the existing unpaved NPS “River Road” just north of the mountain is already patrolled regularly by the Border Patrol,’ Krumenaker said. He added: ‘Let’s not forget, illegal border crossing activity inside BBNP is less than 0.05% of that along the entire border. For this, we are spending $1.7 billion of taxpayer money and irreparably damaging one of the nation’s crown jewel national parks.’
Krumenaker also confirmed that contractor Southwest Valley Constructors is set to begin clearing activity within Big Bend National Park this week to install concrete survey markers, meaning groundwork is already under way even as the road’s route remains disputed.
Peregrine Falcon Nesting Sites at Risk
Mariscal Canyon, a five-mile stretch within the park, supports the highest density of nesting peregrine falcons in Texas. The canyon hosts up to three nesting pairs annually, representing as much as a fifth of the entire statewide population, currently estimated at 15 pairs. Peregrine falcons remain on the Texas state list of endangered species, having spent nearly two decades on the Federal list between 1970 and 1999.
According to the Bird of Prey Conservancy, CBP’s current plans for the area may destroy as much as one-fifth of Texas’ statewide peregrine falcon population, as the agency intends to dynamite one of their major nesting sites in the state. The Mariscal Mountain road plan, if executed as currently mapped, would cut directly through this habitat.
CBP did not respond to a request for comment in time for publishing. Congressman Doggett’s letter, now carrying the signatures of 12 members of Congress, remains the most detailed public challenge to the route and calls on the relevant commissioners to provide full project documentation before any further site work proceeds.
