AS THE story emerges about the criminals President Trump hired and promoted, it is hard to pay attention to a seemingly small announcement last Tuesday from the Environmental Protection Agency, redefining the extent of the EPA’s powers over the nation’s waters. Yet Mr. Trump’s legacy will not just be one of moral ruin in the White House. He and his agents are dramatically curtailing the federal government’s role in fighting pollution, changes that will take years to undo.

The EPA is redefining what bodies of water and hydrological features it can regulate. Features that are “Waters of the United States,” under the EPA’s definition, cannot be polluted or destroyed without a government permit. Everyone agrees that major bodies of water — such as the Chesapeake Bay and the Potomac River — qualify for protection. But what about the tributaries that feed the bay and the river? What about the streams that, in turn, feed them, or the adjacent wetlands? And what about the channels that interact with the system through flooding, storm runoff and groundwater connections?

Attempting to settle these long-running questions, President Barack Obama’s EPA took an expansive view of the agency’s jurisdiction, recognizing that large bodies of water are only as good as the water that feeds them. It might not seem obvious that streams that only run after a storm, or waters that lack surface connections to larger bodies of water, actually affect water quality far elsewhere. But that was the insight that animated the Obama rule. If it behaves like a tributary, Obama EPA officials insisted, it must be regulated.

Not surprisingly, farmers, builders and drillers did not like the rule, and Mr. Trump commanded a rollback almost immediately after entering the White House. The EPA responded Tuesday with a standard that would scale back federal oversight to a level not seen in decades. Ephemeral streams — which emerge only during a storm — would no longer be covered. Wetlands and other features that only have groundwater connections to larger bodies of water would also be exempt. Many man-made channels that flow into larger bodies of water would not be covered.

The new definition would give farmers license to dump fertilizers onto their land with far less care if the pollution makes its way downstream, or coal-mining interests that eviscerate mountaintops and drive the rubble into nearby valleys. Experts have warned with increasing alarm that this mountaintop-removal mining harms water quality and fish populations — and there is evidence it hurts people living nearby, as well.

The EPA insists that it is simply following the law as written. Yet there is a good case that the agency’s narrow view of its authority may, in fact, be illegal. The Supreme Court has offered mixed guidance, but the closest the court has come to a bottom line is this: The federal government has authority over large, navigable bodies of water and those with a “significant nexus” to them. That gives the EPA broad authority to recognize that, say, water flows below ground as well as above it.

Defining the waters of the United States has been a complex, years-long process, and the dispute is long past due to be settled. But not how the Trump administration would settle it.