Nice, but brief, article in the Guardian (UK). And nice picture if you hit the link.
The river was a raging sewer when Seeger set out to save it in the 1960s, a liquid dump for industries that grew along its banks, full of PCBs from the electrical industry, sewage discharges, pesticides, and other contaminants. The main traffic was cement and oil barges. The public largely stayed away.
Seeger, with his late wife, Toshi, built his own 19th-century wooden sloop, the Clearwater, and as he sailed the river, he began asking commercial fishermen to work with him to bring the river back.
The boat would later turn into an environmental organisation, which remains active today.
And remarkably, the effort to save the Hudson worked. Under public pressure, PCBs were banned in the 1970s. In the early 1980s, the Environmental Protection Agency designated a 200-mile stretch of the Hudson as a clean-up site. In 2001, the EPA embarked on another monumental project to dredge the river for sediment contaminated by PCBs. That project is ongoing.