For decades, the Supreme Court struggled to define a key term at the heart of the Clean Water Act, the landmark 1972 legislation that forms the backbone of America’s efforts to “restore and maintain the chemical, physical, and biological integrity of the Nation’s waters.”

It’s an admittedly difficult question, that is now in the hands of the most conservative Supreme Court since the 1930s. And the Court’s Republican-appointed supermajority seems poised to deal a severe blow to the clean water law, in a case that could do significant harm to America’s efforts to prevent floods and to ensure that everyone in the country has access to safe drinking water.