After living in Vietnam / Cambodia / Laos for 4 years, I came to the conclusion that there is no hope to the problem.

It is literally in the local culture to just throw trash into the gutters. You can pick up as much as you want, but there will be 1000x more thrown into the rivers and flushed into the oceans. Changing the culture of people who are living moment-to-moment on absolutely nothing at all, isn't realistic either.

Depressing AF, but far better to spend our time focused on other things than just trying to clean it up.