The New River, which flows north into California from Mexico, is so polluted with industrial and agricultural runoff that environmental experts treat it like toxic waste, reports CBS News Correspondent Sharyl Attkisson.
"You can become very sick," said Jose Angel, a senior engineer with the California Water Resources Board. "There may be viruses present in the water that basically can be associated with tuberculosis and some other diseases."
But migrants are taking risks like the New River to evade a border patrol crackdown along safer routes. Known as "Operation Gatekeeper," the effort has cut illegal immigration in certain areas using stepped-up manpower and high-powered surveillance techniques.
"To me, it's just unimaginable that anyone would jump into raw sewage, but they do," said U.S. Border Patrolman Manuel Figueroa.
Swimming in the New River is so dangerous even the border patrol is reluctant to follow.
"We don't want out agents to go in that river because every disease known to man is there," said Harold Beasley, acting chief of the El Centro sector of the U.S. Border Patrol. "It's just an operational nightmare for us."
And it's not just the New River.
This year, Arizona's desert became a killing field for 70 undocumented immigrants overcome by heat and thirst.
"I thought we were going to die," said one immigrant who, with a friend, attempted to cross the border through the hot desert. "I told my friend to find a knife so we could end the suffering."
In Texas, two men drowned in the Rio Grande, a tragedy captured on tape by Mexican television.
These deadly incidents and the fact that more Mexicans are risking their lives to enter the United States are forcing the border patrol to change its methods. In Arizona, planes now search for migrants in the desert. And in Texas, agents take river rescue classes.
"Our job is to apprehend. But our job is also a very humanitarian job," said Beasley. "If there are people out there who need help, we're going to seek them out."
But immigration advocate Roberto Martinez blames "Operation Gatekeeper" for over 500 deaths since it began in 1994. He says instead of stronger border enforcement, the United States should do more to address Mexico's stifling poverty.
"'Operation Gatekeeper' has created one of the worst human rights tragedies in the last hundred years," said Martinez of the American Friends Service Committee.
"We have an immigration population that's so desperate to get to the United States, they're willing to risk their lives to get jobs here. Let's face it, here they can make $4 to 5 an hour, but in Mexico, it's maybe $3 to 4 dollars a day."
Newly elected Mexican president Vicente Fox vows to change that euation by turning Mexico's economy around. But that could take years. In the meantime, border patrol agents expect the nightly exodus along the toxic waters of the New River to continue.
"People are going to come to the U.S. no matter how good your enforcement agency is, no matter how many agents you put out on the border," said Beasley. "People are still going to be desperate and they're going to come to the United States."