There are a lot of unknowns when it comes to the monumental amount of plastic in the ocean, including how exactly it gets there in the first place. The folks at the Ocean Cleanup Project have just conducted a study that explores one of the major inputs, the world's river systems, with the findings suggesting that these waterways funnel millions of metric tons of plastic waste into the oceans each year.

A 2015 study published in the journal Science found that somewhere between 5 and 12.7 million metric tons of plastic waste enters the ocean each year, spilling out from leaky landfills, open dumps or simply good old-fashioned litter. Collecting what is already out there is part of The Ocean Cleanup's modus operandi, but any real long-term solutions will require a better handle on how it arrives at sea to potentially collect it closer to the source.

To that end, its latest study set out to quantify not just how much plastic pollution is entering the ocean through the world's rivers, but how the amount varies in between the individual waterways. The team did this by gathering data on population density, waste management, topography, hydrography and dam locations. This was then calibrated against field measurements of plastic flow from different rivers and used to form a model of yearly plastic input from rivers into the ocean.

According to the team, somewhere between 1.15 and 2.41 million metric tons of plastic wind up in the ocean each year by way of rivers. Its analysis looked at a total of 40,760 ocean-bound rivers, yet the researchers say that two thirds of the total input can be attributed to just 20, found mostly on the Asian continent.

Their modelling also shows that the input varies a lot between seasons, which the researchers say is the result of debris washing into main waterways from river banks and creeks. Three quarters of the total annual input enters the ocean between May and October.

The team has worked its data into an interactive map, which can be accessed online, while the study was published in the journal Nature.

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