he child-sized blue jeans lay twisted and forlorn in the scrubland along one of the most popular routes for undocumented migrants crossing from Mexico into Texas.

Chris Cabrera surveyed the scene from his white pickup truck. A border patrol agent for 13 years, he knows how to spot the clues, some obvious – like the jeans – others more subtle, like the flattened grass nearby that formed a northwards path through dense bushes.

Its width suggested two or three people walking side by side, which Cabrera said was an indicator of drug smuggling activity: migrant groups tend to move in single file.

"In a week or so that'll be a really good trail," he said.

Only a couple of hundred yards away, cars rushed along the Anzalduas international bridge, gateway to one of several legitimate ports of entry in the area.

But spring and summer are peak seasons for crossings by other means. A couple of minutes earlier a border patrol van drove under the bridge along a bone-jangling rutted single-track path, carrying 13 women and children from Guatemala and Honduras who had turned themselves in to border patrol agents.

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"Every day we're getting more women and children than the day before," said Cabrera, 41, a local border patrol union representative. He estimated that 60% of those apprehended are turning themselves in.

It is almost a year since a surge in crossings by unaccompanied Central American children overwhelmed local processing and holding centres and put the Rio Grande Valley at the centre of a humanitarian and political crisis.