It is estimated that approximately 80% of ocean plastics come from land-based sources with the remaining 20% from marine sources such as fishing gear. Plastic ends up in the ocean mostly from land, flowing downstream through rivers to the sea. This plastic waste is produced by littering, improper waste disposal and illegal dumping of waste.

Nature reports that the most polluted rivers in the world are the main factors responsible for plastic being carried to the seas. Four of these rivers are on the Asian continent; the fifth is in Africa.

https://www.iberdrola.com/

It is estimated that 1.15 to 2.41 million tons of plastic enter the oceans from rivers like these. Most of the plastic pollution in the sea comes from China, Indonesia, the Philippines and Vietnam.

Poor waste management of plastic leads to dumping of plastic in these rivers. Every year, 10 million tons of plastic are dumped into our oceans. The United Nations Environment Program predicts that the amount of plastic waste entering aquatic ecosystems could nearly triple to a projected 23–37 million tons per year by 2040. It has been said that by 2050, plastic will outweigh all the fish in the sea.

From aquarium.ucsd.edu