Earth's oceans have a big plastic problem. They receive roughly 8 million metric tons of plastic waste every year, much of which can drift around for decades or centuries without truly decomposing. Instead, it just crumbles into smaller pieces known as microplastics, which often fatally trick marine wildlife into eating them.

Some ocean plastic is discarded directly into the ocean — from sources like cargo ships, fishing boats and oil rigs — but a large amount washes there from shore, including inland litter carried to coasts by rivers. In the Great Pacific Garbage Patch, for example, about 80 percent of debris began its journey as terrestrial trash.

Like the plastic itself, any solution to this problem will need to come from people all over the planet. That said, some places have more room for improvement than others. According to a new study, rivers carry up to 4 million metric tons of plastic out to sea per year — but just 10 rivers may deliver as much as 95 percent of it.

plastic pollution in a river Discarded plastic bottles congregate near a river bank in Chaiyaphum, Thailand. (Photo: Bubbers BB/Shutterstock)

Eight of those 10 rivers are in Asia, the study found, similar to the findings of another recent study on plastic pollution in rivers. This also fits with earlier research on plastic pollution by country, which has linked the problem with factors like population density and waste-management infrastructure. According to a 2015 study, 11 of the top 20 countries for plastic pollution are in Asia, with China at No. 1. Other countries in the top 20 include Brazil, Egypt and Nigeria — as well as the U.S. at No. 20.

For the new study, published in the journal Environmental Science & Technology, researchers analyzed dozens of previous studies on plastic in rivers. This covered 79 sampling sites along 57 rivers around the world, and showed that a river's plastic load is positively related to mismanagement of plastic waste in its watershed.

The top waterway for ocean plastic seems to be China's Yangtze River, which carries as much as 1.5 million metric tons of plastic into the East China Sea every year. The Yangtze is Asia's longest river at 6,300 kilometers (nearly 4,000 miles), and passes through major population centers like Chongqing, Wuhan, Nanjing and Shanghai, the most populous city in China with more than 24 million people.

Yangtze River in Chongqing, China A view of the Yangtze River in Chongqing, China. (Photo: Fred Dufour/AFP/Getty Images)

The Yangtze showed the highest load of microplastics seen in any river, while the San Gabriel River in Los Angeles had the highest specific loads of macroplastics. The concentration of plastic waste varies widely from river to river, as it does within the ocean, but the average river concentration "is roughly 40−50 times higher than the maximum concentration observed in the open ocean," the researchers write.

Here are the top 10 river systems contributing to ocean plastic, according to the new study, as well as the seas they feed and the continents where they're located:

  • Yangtze River, Yellow Sea, Asia
  • Indus River, Arabian Sea, Asia
  • Yellow River (Huang He), Yellow Sea, Asia
  • Hai River, Yellow Sea, Asia
  • Nile, Mediterranean Sea, Africa
  • Meghna/Bramaputra/Ganges, Bay of Bengal, Asia
  • Pearl River (Zhujiang), South China Sea, Asia
  • Amur River (Heilong Jiang), Sea of Okhotsk, Asia
  • Niger River, Gulf of Guinea, Africa
  • Mekong River, South China Sea, Asia

While ocean plastic remains a daunting problem, this could be good news for the quest to control it. These 10 waterways contribute between 88 and 95 percent of the total plastic load that oceans receive via rivers, the study's authors conclude, so they would be good places to focus our efforts on better waste management.

"The high fraction of a few river catchments contributing the vast majority of the total load implies that potential mitigation measures would be highly efficient when applied in the high-load rivers," the researchers write.

"Reducing plastic loads by 50 percent in the 10 top-ranked rivers," they add, "would reduce the total river-based load to the sea by 45 percent."

albatross chick on Midway Atoll An albatross chick stands amid garbage on Midway Atoll in the North Pacific. (Photo: Kris Krüg/Flickr)

That would be huge, so it certainly makes sense to focus on these 10 watersheds. Still, this study doesn't absolve people living elsewhere. Even small amounts of plastic waste can kill marine wildlife, including already-endangered animals like sea turtles. And while industrial nations have gotten better at managing plastic waste overall, their failures are still significant, especially near their own coasts.

Some of that plastic pollution comes from less obvious sources, like synthetic fibers or toothpaste, but as study author Christian Schmidt tells iNews, much of it boils down to one of the most basic environmental gaffes of all: littering. "The main source in developed countries is littering," says Schmidt, a researcher at the Helmholtz-Centre for Environmental Research in Germany. "This could be reduced if, for example, people would stop throwing food packaging out of their car windows."

That may seem obvious, but it's easy to overlook the many ways we use — and discard — plastic throughout the day. And given the ecological problems it can cause wherever it ends up, it's almost always worth the effort to prevent even a little plastic waste. For inspiration, check out this list of ways to keep plastic where it belongs.

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.