In a mission to clean up trash floating in the ocean, environmentalists pulled 40 tons (36 metric tons) of abandoned fishing nets this month from an area known as the Great Pacific Garbage Patch.
https://www.voanews.com/science-health/40-tons-fishing-nets-...
40 tons in a month by an NGO funded by donations and sponsors.
[1] https://www.theguardian.com/environment/2019/nov/06/dumped-f...
Regulation in this area would be a win-win for almost everyone involved.
Another 9 million tons (8 million metric tons) of plastic waste, including plastic bottles, bags, toys and other items, flow annually into the ocean from beaches, rivers and creeks, according to experts. So 8,000,000/year vs ~36 per month.
In other words they spent 300,000$ and reduced the oceans added plastic load that month by 0.0054%.
So basically, what the Ocean Cleanup project is also already doing with their Interceptor systems? https://theoceancleanup.com/rivers/
My suggestion would be two different lines each collecting from over half the river. One upstream and one down so boats can still easily navigate the channel and 24/7 365 operations are cheap. Further, you need a system designed to operate in floods when the majority of plastics are washed out to sea.
Maybe it is correct to think of 40 tons as not a big dent in the problem, but I'm not sure.