Millions of tons of plastic waste enter the world's oceans every year. This pollution, when broken down into tiny bits called microplastics, can damage the health of marine life. Cleaning it all up would be impossible, but stemming the tide could help reduce the potential harm. To do this, however, researchers need a better understanding of how plastic makes its way into the oceans in the first place. Rivers, which flow from inland areas to the seas, are major transporters of plastic debris. But the concentration patterns aren't well known. Christian Schmidt and colleagues wanted to fill in this knowledge gap.