Potomac Water is a Health Hazard (1973)
Remember how nasty the Potomac River was? Yes, it’s still not the greatest, but it’s swimmable in some parts (as long as you don’t have open wounds). Back in the 1970s, it was absolutely polluted. Check out this sign from 1973, recommending you seek immediate medical attention after prolonged exposure.
Source: U.S. National Archives