"Almost all plastic in the ocean comes from just 10 rivers" [0] - The source is Deutsche Welle, which appears to be Germany's equivalent of the BBC. [1]
"It turns out that about 90 percent of all the plastic that reaches the world's oceans gets flushed through just 10 rivers: The Yangtze, the Indus, Yellow River, Hai River, the Nile, the Ganges, Pearl River, Amur River, the Niger, and the Mekong (in that order).
These rivers have a few key things in common. All of them run through areas where a lot of people live — hundreds of millions of people in some cases. But what's more important is that these areas don't have adequate waste collection or recycling infrastructure. There is also little public awareness that plastic trash is a problem at all, so a lot of garbage, gets thrown into the river and conveniently disappears downstream."
Someone linked a great map of these 10 rivers in a past thread, but I can't seem to find it.
[0] http://www.dw.com/en/almost-all-plastic-in-the-ocean-comes-f... [1] https://en.wikipedia.org/wiki/Deutsche_Welle