RIO BRAVO, Texas - In theory, the Border Patrol could prevent the flow of undocumented aliens into the U.S through a combination of high-tech approaches such as electronic sensors and a heavy presence of agents on the ground.

I discovered the limits of that theory as I was poking around the town of Rio Bravo on Tuesday with Andre the photographer.

As we pulled up to the banks of the Rio Grande in this small town near Laredo, we saw three Border Patrol vehicles stationed so the agents could keep an eye on the banks of the river.

But that’s not easy, especially not in this spot. Upstream a bit there’s a long, heavily vegetated island blocking the view of the Mexican bank of the river.

A migrant - or party of migrants – can make it to the island unseen and then just wait for a chance to make the short sprint across the small channel separating the island from the United States.

The bank is steep in most spots, but there’s one culvert that drops down to river level. We figured we’d wander down there to get a closer look.

As I poked around, Andre started shooting a video of the scene.

Then something totally startling happened. An inflatable boat packed with people emerged from the reeds just a few yards from us. Apparently they’d mistaken us for border agents.

There were eight people in the boat, but they had just one oar. That didn’t seem to slow them down. It took a mere 45 seconds for them to make it back to the island, the video showed. At that point, the river is so shallow they got out and walked back to Mexico.

Before long a Border Patrol agent drove up. We got to chatting. Like the rest of the agents we met, this guy was very affable and informative.

When we told him about the incident, he said it was nothing out of the ordinary. The occupants of these boats will hide in the reeds all day and then try to blend in with the townspeople at night.

That’s not that difficult. We saw that earlier in the day when we went to the river banks in the city of Laredo. This is in the epicenter of illegal entry, not far from McAllen, where President Trump made his appearance Thursday.

For hundreds of miles in this region there is no border wall – or fence - or any obstacle other than the Rio Grande.

This has its advantages. We learned that when we were standing just under the international bridge that connects Laredo with the Mexico town of Nuevo Laredo.

A pickup pulled up. Out came a Mexican-American guy by the name of William Turgeon. Out also came a whole lot of fishing gear.

You can catch some pretty big fish in the Rio Grande, Turgeon told us. He also told us he was a big supporter of Donald Trump’s border policy – even though he has lots of relatives living on the other side of the river.

Turgeon told us he was born in the U.S.A. and had spent a lot of time in the Marine Corps.

“You cannot just come in and insist you’re a citizen,” he said.

He told us he’d seen plenty of people swimming over from Mexico right here at the most populous spot on the Rio Grande.

“I’ve seen them come right here – right in front of here,” he said. “They’d be swimming like it was nothing.”

As to whether a wall was the best solution, Turgeon said, “You’ll never stop people unless there’s a wall.”

But he also noted that once a wall went in, there would be no more fishing. The pleasant park with picnic tables where we were talking would be up against a metal barrier.

When we got talking to a Border Patrol agent there, I asked him how often people cross.

“Pretty often,” he answered.

The wall/fence that President Trump proposes would stretch for 45 miles in the Laredo area. Building it wouldn’t be easy. We saw that as we drove along the river, checking out different spots along the way.

The river takes a lot of twists and turn and forks off at various estuaries and lakes. I don’t envy the planners who’d have to figure out where it would go.

There are some spots where a fence seems to make perfect sense. One was the small town of Roma. We stopped there because we wanted to cross over into Mexico to take a look at a border town from the other side.

We crossed into the town of Miguel Aleman at the same time as a big bunch of schoolkids. They live in Mexico but go to school in America, a symbol of how closely intertwined these border cities are.

The vast majority of crossings are perfectly legal, with Mexicans heading into America to shop at outlet malls and Americans traveling to Mexico for leather goods and other bargains.

But even here, the Border Patrol is stressed. As we got back to the American side, we heard what sounded like gunshots. A nearby Border Patrol agent started up his truck and zoomed off.

At the base of the bridge on the American side, we saw three deflated inflatables lying in the dirt. There was a lot of clothing in the reeds, discarded by crossers who switch to dry clothes to blend in with the population.

This was one spot where a wall/fence would seem appropriate. Border Patrol agents told us the role of a fence is not so much to stop all traffic but to channel it and slow it down to the point where they can respond.

They’re well-prepared for that. As we drove along the highway that winds along the river for hundreds of miles, we saw quite a few unmanned surveillance blimps tethered to the ground.

Border Patrol trucks seemed to be as common as cabs in New York City. They’ve got a navy as well, as we saw when a patrol boat came down the river shortly after we’d surprised those eight people in Rio Bravo.

The agents have their work cut out for them. I learned that when I asked the agent in Rio Bravo what would likely happen with those eight people we had surprised.

“They’ll be back,” he said.