Rose, good to see you back. When I was a kid in Pittsburgh, one of its three rivers that I lived near was the Monongahela. At that time, the rivers were polluted by the steel mills that ran up and down the "Mon Valley." The only fish that we ever caught were catfish and carp.
As the rivers got cleaner, other fish began to appear. Today, the rivers are mostly with a diverse number of fish species.
The tradeoff? Nearly every mill has shutdown, and in the 1980s, hundreds of thousands of people lost their jobs.