McALLEN, Texas -- Carlos Gonzalez gunned the throttle. The roar of the outboard motor warned everyone within a mile that the U.S. Border Patrol was rounding the serpentine bend of the Rio Grande known as the cola del diablo, or devil's tail.
Turtles scurried off logs and into the green, murky water as the Border Patrol boat raced by at 35 mph, flanked by a second boat with a hard-faced agent in dark sunglasses holding an M-4 carbine assault rifle.
On the Mexican side, men with fishing poles by their feet rested in the tropical sun. Their eyes followed every move the boats made. Gonzalez, a 20-year veteran of the Rio Grande beat, suspected that most were lookouts who would take out cell phones and alert smugglers when the coast was clear.
He cut off the motor and coasted into a foul-smelling landing less than half a football field across from Mexico, where countless feet had carved a well-worn path up the reedy riverbank. It was littered with inner tubes, plastic bags and underwear - unmistakable clues that immigrants continued crossing into Texas. But when the border agents went ashore, the beachhead was still. The only signs of life were swarms of buzzing mosquitoes.
"We're never going to stop them - never," Gonzales said. "This was happening before I was born, and it will be happening long after I am gone. There is no way to shut the river down."
Though politicians and regular folks all over America agree that stopping illegal immigration starts with sealing the border, agents on the front line are under no illusions that they can catch everyone - even with the 6,000 extra agents that President Bush plans to put on the border over the next two years.
The challenges of policing the southernmost boundary with Mexico were clearly visible recently as Gonzalez and two other agents fanned out on different shifts across the Rio Grande Valley sector, a stretch of 18 counties that ends where the river - feared for its murderous whirlpools and undertow - calmly empties into the Gulf of Mexico.
The 1,500 agents in this region are mainly small-town men from South Texas. They pride themselves on working hard each day to stop hardened criminals and terrorists, and they catch as many illegal immigrants as possible. But after years of experience on the Rio Grande, they realize that desperate people will always find a way across, as long as opportunities on one side remain so much better than on the other.
Equipped with all-terrain vehicles, mountain bikes, horses, helicopters, firearms, motion sensors and infrared cameras, the agents cast a sweeping dragnet over 17,000 square miles.
In addition to keeping vigil along the riverbank and staffing highway checkpoints to inspect cars and trucks heading north, agents board buses to question passengers riding to big cities in Texas. They double-check identification at regional airports. They stop freight trains and search cargo holds for immigrants riding the rails. They comb barrier islands for drug runners sneaking up the coast. And they scour the mesquite and brush of cattle ranches for smugglers cutting around the highway checkpoints.
The Rio Grande was one of the first places where the government got serious about fighting illegal immigration, and the Border Patrol says a steady buildup of agents and equipment has reduced drug running and human smuggling here over the past two decades. It has also improved the lives of hundreds of local men and women, who have found careers in the Border Patrol.
Many agents readily acknowledge that they did not join the Border Patrol because of a passion to stop illegal immigration. They were drawn to the job because it pays better than other lawful jobs in the region and promises a life of action along the untamable river Mexicans call Rio Bravo.
"I was looking for a career," said Derek Conrow, 29, an agent from El Paso who was Gonzalez's boat mate. "If you like the outdoors, this is the place to be. We're not stuck in the office all day."
Agents generally earn $51,972 a year after three years - twice the median household income in this region - and average about 25 percent more in overtime pay.
Gonzalez stood on the dock at Anzalduas County Park and looked out at another dock, in another park, in another country.
On weekends, each park fills up with Latino families picnicking as children wade in the river. When the agents aren't around, smugglers on Jet Skis scoot illegal immigrants from one side to the other in seconds, Gonzalez said.
He pushed off the dock and resumed his day patrol along the stretch of the Rio Grande southwest of McAllen, where the Border Patrol has a regional office. The 18-foot fiberglass boats are supposed to serve as deterrents. But they also make the agents, in white polo shirts with "Border Patrol" emblazoned on the back, highly visible targets.