On Tuesday, in the Trump administration’s latest assault on the environment, the Environmental Protection Agency proposed rolling back Clean Water Act protections that have helped make America’s rivers and streams fishable and swimmable, in the process threatening drinking-water supplies across the country.

Once again, the E.P.A. is disregarding basic science. This latest proposal obscures its harmful effects with legalese that draws dubious distinctions between certain streams even though pollution flows downstream regardless of the legal terminology the agency deploys. This is a thinly veiled effort to slash water pollution protections that have long been embraced by both Republican and Democratic administrations.

President Trump is once more playing to his base, this time to rural communities fearing greater regulation of the agricultural runoff that each year creates worsening dead zones in the Great Lakes and the Gulf of Mexico. Farmers do face significant challenges complying with environmental requirements. But in purporting to address their concerns, the E.P.A. would place at risk one of the greatest American environmental success stories: the transformation of our rivers and streams from open sewers in the 1960s and 1970s to far healthier waters today.

Congress passed the Clean Water Act in 1972 during a Republican administration, just three years after the Cuyahoga River caught on fire in Cleveland and an oil spill coated beaches in Santa Barbara, Calif. Five years later, in 1977, the E.P.A. and the Army Corps of Engineers, which oversees the regulation of wetlands, determined that the Clean Water Act protected all rivers and streams, as well as their tributaries, even if those waterways flowed only during certain times of the year or after it rained or snowed. The former are known as intermittent streams, the latter, ephemeral streams.