A U.S. surveillance helicopter hovering over the Rio Grande spots a body floating near a muddy bank on the Mexican side of the river.

Soon another body turns up, then another. A Mexican investigator arrives and holds up his hand confirming the grim tally: four men and a woman.

The grisly discovery last month is part of a spike in drownings since October. Immigrants, desperate to avoid detection at a time of increased patrols, are choosing more dangerous and remote crossings into South Texas. The Border Patrol has responded by expanding its search-and-rescue teams to monitor the area, particularly weed-choked irrigation canals where many of the bodies are being found.

"The canals and areas of the river they are trying to traverse, they typically weren't trying to go across before," said Raul L. Ortiz, deputy chief of Rio Grande Valley sector.

Encompassing some 320 miles of river, his sector has already seen at least 16 drownings in nearly six months, nearly a third of them in the canals. The tally is only five short of the number of deaths reported from October 2013 to September, when a historic surge of immigrant women and children were crossing into South Texas. Though illegal crossings have decreased dramatically from last summer, more law enforcement officials are patrolling the border to deter another wave of immigrants.

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Many of the bodies are being discovered just southwest of Mission, where the fire department's dive-and-rescue team has had a busy winter. In January and February alone, it recovered at least six bodies in the murky canals.

"It used to be one a month," Mission Fire Chief Rene Lopez Jr. said. "Now it's one a week."

Some canals are 50 feet wide with steep brush-covered embankments that make it hard to climb out. The waters look deceptively calm but currents run through them, and swimmers often get caught in hydrilla, an invasive water plant, or debris such as shopping carts and tires.

"They get tied down and it's hard to get away from that in black water," said Capt. Joel Dominguez, part of the rescue team. "And they are often panicking, running from agents."

To provide help, the Border Patrol has transferred eight members of an elite rescue unit from El Paso to the Rio Grande Valley, bringing the total number of agents to 30. They're trained in swift-water rescues, emergency medicine, tracking and diving.

But experts say the extra resources can only do so much to prevent deaths in the Rio Grande, which has long claimed the lives of migrants trying to cross it.

"It's not like slow dying in the desert or in the prairies of South Texas," said Nestor Rodriguez, a University of Texas at Austin sociology professor who published a study that found drowning was the most common cause of migrant deaths. "A drowning happens in a minute or two and it's much more difficult to save someone."

Border Patrol trucks and Texas state trooper vehicles stand watch daily on the levees that separate the Rio Grande from the canals. Smugglers are now quicker to abandon their groups in the river or scrubland, according to Felix Cantu, the Border Patrol agent in charge of McAllen station.

Until recently, the smugglers "usually would take the group all the way to where they are going," he said. "Now they are trying to get more separation, remove themselves form the group, obviously not to get caught."

The dead will often float to the surface within a few days, still wrapped in their packs. Fish and turtles often have fed on the bodies, making identification difficult if the person didn't carry a form of ID. Three of the five bodies discovered last month by the helicopter crew were identified within days: two Mexicans and one from El Salvador.

"You just feel for them," Lopez Jr. said. "They are young, in their 20s and 30s, even teenagers."

Guatemalan Consul Allan Perez has had the unenviable task of notifying families back home that their loved ones died. Just last month, the decomposed body of a 27-year-old Guatemalan man was found in one of the canals.

He said he impresses upon immigrants who survive but get deported that they are the lucky ones.

"I tell them you are returning without money, injured, with broken dreams and your head down," he said. "But you are going back alive."

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