Patrolling the Rio Grande for migrant crossings: Reporter's Notebook

I was out with Texas State troopers onboard a tactical marine boat as they patrolled the Rio Grande on Friday.

Staff Lt. Ivan Tijerina told me and those I was with that the troopers haven't seen a surge in migrants like the one underway now in years and, notably, he's never seen so many unaccompanied minors crossing the border.

"We've seen everything from empty one-gallon jugs made into makeshift rafts to 55-gallon drums. They make them into makeshift rafts -- mattresses, you name it. Whatever can float," he said.

Crossing the river is perilous and sometimes deadly. In 2019, a viral photo of father Óscar Alberto Martínez Ramírez and his toddler daughter Valeria lying face down after drowning in the Rio Grande highlighted the risk migrants take.

On March 20, a U.S. Border Patrol Marine Unit was flagged down to help three unresponsive migrants stranded on an island in the Rio Grande. The Guatemalan woman and her 3-year-old son, a Mexican national, survived. But her 9-year-old daughter, also a Mexican national, later died.

On the Rio Grande, I spotted several makeshift rafts tangled in trees along the edge of the river -- troopers hopped on and off, puncturing the rafts so they couldn't be used again.

According to the Missing Migrants Project, 76 migrants have died at the U.S.-Mexico border so far this year. The organization, which tracks deaths of migrants who have gone missing along mixed migration routes worldwide, also reports the top causes of migrant deaths along the border in 2020 include being shot and killed, vehicle accidents and drowning.

"A lot of times smugglers, they're called coyotes -- will bring them to the river and then say 'OK from here, that's it.' It's on your own, you cross and then leave them out here," Tijerina said. "It becomes dangerous for them because they don't know that terrain. Terrain like this is very thick and vegetation, people can get lost and get hurt. You got venomous snakes, you have, you know, animals, and once they get hurt. It's very hard for somebody to get help."

In the past 30 days, immigration authorities at the southern border have stopped an average of 5,000 unauthorized immigrants a day, about half of whom are family members and unaccompanied minors, according to a senior Border Patrol official.

But with the surge in migrants, Tijerina told us some are easily finding a way around authorities.

"One of our main challenges, we'll have some marine units out along areas on the river. The water's too shallow where we cannot patrol, and of course those areas are easily exploited because we can't reach there and depends on the accessibility of the land. That's easy to be exploited and used for human trafficking, as well as narcotics trafficking," Tijerina said.

While we were out on the water, troopers spotted a car with its doors open -- a sign of an imminent crossing. By the time they doubled back, the car left to find another spot on the river that was not as well covered.

ABC News' Benjamin Siegel and Luke Barr contributed to this report.