PRESIDIO, Texas — A levee protecting this western Texas border town from the swelling Rio Grande failed Thursday, and authorities were mobilizing helicopters and inmates for a sandbagging operation aimed at turning railroad trestles into a makeshift dam.

Gov. Rick Perry issued a disaster declaration for Presidio County as water crept toward populated areas of the town of about 5,000.

Presidio County Attorney Rod Ponton said a second levee failure is probable, however, and if that happens "all bets are off" on the trestle plan protecting the town.

"It won't be a wall of water, but it could be pretty fast," he said.

The levee breach Thursday morning happened where water had been flowing over the top of it since at least Tuesday, Ponton said.

A golf course and farmland have been swamped, but no homes were immediately threatened, he said. Low-lying parts of the town remained under evacuation orders.

Ponton said Perry's office had agreed to send helicopters to drop large sandbags around a railroad trestle, and that 170 prisoners from a low-security facility will throw smaller sandbags on two other trestles. The help was to arrive Thursday evening, and Ponton said work would continue through the night.

Crews worked Thursday evening to push tons of dirt beneath one of the railroad trestles, where water was steadily flowing out. They also worked to bolster a stretch of railroad track that forms a berm that abuts the levee nearby. Meanwhile, water was seeping beneath the trestle on which helicopters will drop sand bags.

Presidio, across the Rio Grande from Ojinaga, Mexico, has been watching the river for two weeks. Heavy rains and the forced release of water from the flood-stricken Luis Leon Reservoir in Mexico filled the Rio Grande to the top of the levee.

Homes and some buildings in Ojinaga are reportedly under as many as 10 feet of water because of a levee breach on that side of the border, officials have said.

Ponton said Presidio officials asked Perry's office to contact the U.S. State Department and request that Mexico stop the release of water. As it stands, Ponton said he expects the release to continue for at least another week.

"This is not a natural disaster. It's a manmade disaster," Ponton said, criticizing river management by Mexican officials.

The U.S. and Mexican leaders of the International Boundary and Water Commission and two others died in a plane crash on their way to check out the flooding. The wreckage was found Wednesday, two days after the plane disappeared.