Another big problem is that rivers in the developing world aren't particularly well controlled. Just this summer, a massive flood in the Yangtze basin displace

Another big problem is that rivers in the developing world aren't particularly well controlled. Just this summer, a massive flood in the Yangtze basin displaced over 60 million people and inundated a number of large cities. The flood carries anything that will float downstream, and eventually dumps them in the ocean. As it happens, plastics tend to float pretty well. So even if the Chinese government declared that they will shoot anyone who throws trash in the rivers effective immediately, the rivers themselves come into people's homes, snatch plastics, and carry them into the ocean every year.