The Cuyahoga River is “clean” today because all the steel mills and heavy industry are gone in Cleveland.
That may be part of it, but not all. Still a lot of industry up there. Ohio use to have high sulfur coal, well still does, but they built scrubbing plants to remove the sulfur to reduce pollution. I grew up in Zanesville. We used to go to football games in Philo. They had a big coal fired power plant there. I remember every house was a grimy grey from the soot. Not so anymore. Cleaner coal, better filtration,...... Need a lot of money to do that. Take a look at Eastern Europe after the wall came down. An environmental disaster. It is the same in all poor countries.