German here. You can absolutely remediate heavily polluted rivers. Best example might be the Rhine. It was so toxic in the 70s that everything in it died. Now it's a nice river with lots of living things and you can absolutely swim in it. It's still dangerous because of ships and currents, but at least the water itself won't kill you.

But there are still lots of other rivers where remediation takes still place, e.g. because the rivers was used for raw sewage.