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.

(These maps show the journey of plastic waste through rivers to the sea.)

New research published today in Science Advances has