A study published in Environmental Science & Technology 2017 has found that 90% of plastic in the oceans comes from 10 rivers. 8 of whom are located in Asia and the remaining 2 are in Africa.
World Economic Forum has written about this in the article: 90% of plastic polluting our oceans comes from just 10 rivers.
The rivers in Asia are; the Yangtze, Yellow River and the Hai He (all three end in the Yellow Sea); Indus (ending in the Arabian Sea), Ganges (ending in the Bay of Bengal), Amur (ending in the Pacific Ocean) and Pearl River and Mekong River (both end in the South China Sea).
The rivers in Africa are the Nile (which ends in the Mediterranean Sea) and the Niger (which ends in the Atlantic Ocean).
The plastic from these rivers, like other garbage in the oceans, ends in gyres due to ocean currents. In these gyres or patches there is a higher concentration of plastic than elsewhere.