The wall will cause flooding trouble on Mexico’s side

As the White House pushes Congress to fund President Trump's U.S.-Mexico border wall, a new wrinkle has emerged that could stymie parts of the massive project.

Mexican engineers believe construction of the border barrier may violate a 47-year-old treaty governing the shared waters of the Rio Grande. If Mexico protests, the fate of the wall could end up in an international court.

Antonio Rascón, chief Mexican engineer on the International Boundary and Water Commission, said in an interview with NPR that some border wall proposals he has seen would violate the treaty, and that Mexico would not stand for that.

"A concrete wall that blocks trans-border water movement is a total obstruction. If they plan that type of project, we will oppose it," he said in his first public comments on the border wall.

The U.S. Department of Homeland Security has not finalized what the new wall would look like, or precisely where it would go. But some of the initial schematics show it could be as high as 30 feet and made of solid concrete.

Mexico is watching with growing alarm as Homeland Security moves ahead with its plan to dramatically extend a border barrier that already has caused serious flooding.

While Congress debates whether to include funding in a stopgap spending plan, the department has solicited bids for prototypes. Trump also said he would continue to ask Mexico to pay for the wall, but Mexican President Enrique Peña Nieto has said repeatedly that he has no plans to foot the bill.

The 1970 Boundary Treaty lays out the precise border between the U.S. and Mexico and sets rules for the riverside regions. The treaty states both U.S. and Mexican officials on the International Boundary and Water Commission, or IBWC, must agree if one side wants to build any structure that would affect the flow of the Rio Grande or its floodwaters.

The U.S. already has built nearly 700 miles of security fence, and Mexico has consistently opposed it. After Mexico protested the earlier fence construction, the U.S. made some design modifications, but the project went ahead over Mexico's objections.