"The Rio Grande, what side of the river are you going to put the wall? We're not going to put it on our side and cede the river to Mexico," Zinke told members of the Public Lands Council, as quoted by E&E News. "And we're probably not going to put it in the middle of the river."
A 1970 boundary treaty mandates that structures along the Rio Grande and Colorado River, which define the U.S.-Mexican border, cannot disrupt the flow of either.
Zinke conceded that electronic defenses may be more appropriate than a physical barrier in some areas, and that regions with preexisting natural barriers may not need further construction.
"The border is complicated, as far as building a physical wall," he said.