The Choqueyapu River used to be the town's treasure, filled with the gold that five centuries ago attracted the Spaniards to this basin in the Andes.
Precious metals are nowhere to be found near the Choqueyapu these days. Human feces, dead dogs and noxious chemicals fill the waters as they wind through La Paz, bringing disease and overpowering smells to those unlucky enough to live near its banks.
"The Choqueyapu River is no longer a source of pride for La Paz," said Fernando Cajias, a historian. "It is now just a dead, dirty river that shows the failures of this city."
Not only is the river dead, it's deadly. In late February, a fierce rainstorm combined with inadequate drainage systems to swell the Choqueyapu out of its banks, sweeping away dozens of homes and drowning more than 60 people.
The tragedy could have been prevented, city planners say. Officials have known for years about the flooding dangers from a fast-flowing river that drops 2,000 feet from its source 14,000 feet above sea level to the city, which lies in a basin where many rivers meet.
Although the city turned stretches of the Choqueyapu into canals after a deluge in the 1930s, flooding is still common during the rainy season, which hits La Paz from late November to March.
A $100-million plan to improve flood controls was drawn up years ago but, as is often the case in South America's poorest nation, there was no money for it. The city is hoping for outside donations so that it can finally begin putting the ideas into practice.
"We've always felt the importance of implementing the plan, but there simply haven't been the resources," said Guido Capra, a city councilman and engineer. "But there didn't have to be so many deaths [during the February flood]. This plan needs to be a priority, now."
What will probably take even longer to deal with is the contamination filling the river--a tragedy in itself, said Waldo Vargas, the city's environmental director.
"This river is born out of a glacier," he said. "Its waters are impeccable. There are various categories of water, and this river starts with the highest."
But by the time the waters reach downtown La Paz, they have dropped from class A to class E--the filthiest, Vargas said.
The river barely stands a chance as it winds about 25 miles through La Paz, encountering all sorts of contamination and pollution.
Small mountain communities near the river's source muddy the water while extracting peat to sell to gardeners. Farther along, people dig out sand and rocks to sell for construction.
But the most serious trouble begins on the outskirts of the city, where construction workers regularly dump unwanted bricks or cement.
Then comes La Paz's industrial zone, where textile factories let their chemicals drip into the river and slaughterhouses toss in animal fat and blood.
One of the worst--and most unlikely--polluters is Aguas del Illimani, the company that provides water to La Paz's 800,000 inhabitants. While purifying the water taken from the Choqueyapu, the company's machinery produces sludge that oozes back into the river.
At this point, the Choqueyapu flows underground, traveling through flood-control channels downtown. Even here, the river is a garbage can. Most apartment buildings above the channels have connected their sanitation systems to the waterway.
Vargas said that of the 500 tons of garbage produced in La Paz each day, at least one-fifth finds its way into the Choqueyapu and other rivers.
"This is a result of a complete lack of education," Vargas said. "To most of the people living here, this river has always been dirty. They don't know what it's like to use a river for recreation instead of as an instrument for transporting garbage."
He has begun a pilot program along a remote section of the Choqueyapu seeking to teach people about the dangers of polluting their own water.
The chemicals and garbage create a stench when the river leaves the tunnels of downtown and flows south, passing by a small community of tent-dwellers who bathe and wash clothes in the filthy river.
La Paz was originally built in the high plains of the Andes, but when the Spaniards discovered gold in the river in the 16th century, it was refounded lower, and the Choqueyapu became a border between the social classes.
Indigenous people who cultivated potatoes--Choqueyapu is "where potatoes grow" in the Aymara language--were pushed to the west side of the river. The Spaniards built their houses and businesses on the east bank.
Today the class divider is elevation. La Paz lies 12,000 feet above sea level and those with money prefer to live downhill, where it's warmer and there's more oxygen.
The poor, many of them indigenous people, live closer to the river's source, in the high Andes, where the air is thin and the weather harsh.
As the river descends, it passes the middle- and working-class districts in the center of La Paz, then the land occupied by La Paz's small wealthy sector at the bottom of the basin. But the rich build their homes away from the river.
Those who must fully contend with the contamination are the rural people living farther south--and often suffering breathing problems and cholera, Vargas said.
Mainly farmers use the river's water--now filled with chemicals and bacteria--to irrigate crops. Their fruits and vegetables go back to city markets and often cause stomach illnesses.
Although people like Vargas dream that the river will one day flow through the city crystal-clear, many are skeptical given the public's apathy and the government's lack of emphasis.
"On a political level, the environment is not considered to be all that urgent," said engineer David Rada, a consultant to the government's environment department. "Building a school pays off a lot more politically than preserving natural resources."