Dr. Waldman is a professor of biology at Queens College and the author of “Heartbeats in the Muck: The History, Sea Life, and Environment of New York Harbor.”Fifty years ago, Congress voted to override President Richard Nixon’s veto of the Clean Water Act.At the time of the law’s passage, hundreds of millions of gallons of raw sewage was dumped by New York City into the Hudson River every day.Health advisories against eating fish from the Hudson remain, but its ecology has largely recovered, thanks to the law, which imposed strict regulations on what could be discharged into the water by sewage treatment plants, factories and other sources of pollution.Cleaner water has made the harbor far more hospitable, and other steps have helped to rebuild life there, like fishing restrictions and the removal of some dams on tributaries in the Hudson River watershed.John Waldman is a professor of biology at Queens College and the author of “Heartbeats in the Muck: The History, Sea Life, and Environment of New York Harbor.”The Times is committed to publishing a diversity of letters to the editor."