The problem with plastic waste just got more complicated—and so did the effort to stanch its flow into the world’s oceans.

Rivers are the primary conduits for plastic waste to the seas. In 2017, two separate groups of scientists concluded that 90 percent of river-borne plastic waste that flushes into the oceans is conveyed by just a handful of large, continental rivers, including the Nile, Amazon, and Yangtze, the world’s three longest rivers. Cleaning up those rivers—10 rivers were named in one study and 20 in the other—could go a long way toward solving the problem, experts agreed.

New research published today in Science Advances has turned that thinking on its head. Scientists found that 80 percent of plastic waste is distributed by more than 1,000 rivers, not simply 10 or 20. They also found that most of that waste is carried by small rivers that flow through densely populated urban areas, not the largest rivers.

Thus, the Yangtze, which traverses 3,915 miles across China and empties into the East China Sea, and was ranked most polluted by plastics, has been displaced by the 16-mile-long Pasig River in the Philippines, which flows through the capital city of Manila, home to 14 million people.

That’s quite a shift. But it speaks to two important issues key to understanding and solving the plastic waste problem. The research underscores the pervasive spread of plastic waste into literally every crevice of the planet, and the need for solutions far more logistically complex and costly than some of the plastics campaign sloganeering suggests. The study also reinforces what marine scientists and other experts have long argued: that the ultimate solution to protecting oceans and freshwater systems is to contain plastic waste on land, where it originates.

Gary Bencheghib, who heads Sungai Watch, a campaign now cleaning up 45 rivers in Bali, says the research from 2017 didn’t make much sense to him.

“The 10-rivers study surprised me more than anything when it came out,” he says. “It wasn’t reinforcing what we were seeing on the ground in Indonesia in the smaller streams. We live in the tropics in a volcanic region where there are literally rivers every 500 meters and they’re all choking on plastic.”

Better data, big changes

Humans have used rivers since the dawn of civilization to carry away their waste. Yet as the plastic trash issue exploded in the last decade, most of the research focused on plastic in the oceans. Analysis of rivers and other freshwater systems has lagged behind. For example, the first full-scale assessment of plastic waste in India’s Ganges River, conducted by the National Geographic Society, concluded just 18 months ago. A similar analysis of the Mississippi River began last month after 100 mayors of cities along the river corridor joined together to sponsor it as a first step toward reducing plastic waste. Japan is conducting a survey to track plastic in both the Ganges and Mekong Rivers.

The new research was based on new modeling and conducted by several of the same scientists involved in both 2017 river studies. They say the data available four years ago was limited, and led to a heavy focus on the size of river basins and population density. In all, the scientists

analyzed plastic waste in 1,656 rivers for the new study.

The new modeling takes into account activity in those river basins, such as the proximity of rivers to coastlines, as well as the effects of rainfall, wind currents, and terrain, including slope, that ease the movement of plastic into waterways. Plastic flows more easily into rivers from paved urban areas, for example, than it does in forests, and travels farther in rainy climates than dry ones. The researchers also took into account the proximity of landfills and dump sites to river banks, and concluded that those within six miles (10 kilometers) of rivers are likely to spill into them.

“One big difference from a few years ago is we don’t consider rivers mere conveyor belts of plastics,” says Lourens J.J. Meijer, the study’s lead author. “If you put plastic into the river hundreds of kilometers from the mouth, it doesn’t mean that that plastic will end up in the ocean.”

The farther plastic waste has to travel along a river, the less likely it will actually reach the seas. On the Seine River in France River in France, for example, plastic water bottles with labels dating to the 1970s have beached themselves along the riverbank.

One of the surprises, Meijer says, is that small rivers on tropical islands carry so much plastic waste, such as in the Philippines, Indonesia, and the Dominican Republic. Likewise, rivers in Malaysia and Central America, which are fairly short, also disgorge heavy concentrations of plastic waste.

“Not always the usual suspects like the Ganges or Yangtze,” Meijer says.

Another finding is how plastic flows into the oceans differ by climate. In tropical regions, rivers disgorge plastic into the seas continuously, while rivers in temperate regions can flush most plastic in a single month, usually August in the rainy season, or single events, such as flash floods.

One storyline from the 2017 studies remains constant: Most of the rivers that transport plastic to the seas are in Asia. Of the first 50 rivers on the new list, 44 are in Asia, a reflection, the authors say, of population density.

“Asia and Southeast Asia are the hot spots, but that could change,” says Laurent Lebreton, a  co-author. “I am a bit concerned for Africa for the decades to come. The population is growing, it is really young, and the economy is getting better so people will buy more stuff.”

A focus on solutions

The research, which underwent a two-year peer review before publication, was funded by The Ocean Cleanup, a nonprofit founded by Boyan Slat, the Dutch entrepreneur whose quixotic $30 million effort to clean up the plastic in the Pacific Ocean turned him into an international celebrity. Both Lebreton and Meijer work for the nonprofit.

Slat’s team has since developed a trash-eating machine called the Interceptor to collect trash from rivers. It is roughly a variation on Mr. Trash Wheel, the googly-eyed trash barge propelled by a water wheel that has been cleaning up the Inner Harbor in Baltimore, Maryland, since 2008 and now leads a fleet of four trash wheels there.

WHAT YOU CAN DO

—Choose reusable straws instead of plastic.

—Have plenty of reusable water bottles at hand.

—Buy groceries in bulk as much as possible.

—Take advantage of recycling programs.

—Keep plastic films and other non-recyclables out of your recycling bin. 

—Join a beach cleanup in your local community.

In 2019, Slat announced plans to mass produce 1,000 Interceptors and deploy them within five years. The pandemic slowed the pace, but several of the devices are at work on rivers in Malaysia, Indonesia, Vietnam, and the Dominican Republic. The challenge, Slat says, is scaling up to meet such an ambitious goal. “It’s not very difficult to address one river,” he says. “It’s very difficult to do ten or one hundred or one thousand.”

George Leonard, chief scientist at the Ocean Conservancy, who was not involved in the study, says the challenges of cleaning up 1,000 rivers, despite the advancements in equipment designed to handle such a chore, calls attention to the message long prescribed by his organization. “We have always said we need to keep plastic out of the ocean in the first place, rather than relying on cleaning it up as a solution. That means keep it out of the rivers, too.”