President Trump’s Environmental Protection Agency on Tuesday announced new regulatory guidelines for what constitutes “Waters of the United States” under the Clean Water Act, rolling back an Obama-era rule that could diminish protections for millions of acres of wetlands, environmental advocates warned.

With this change, federal clean-water regulations will no longer apply to thousands of bodies of water in the United States.

“While the Trump administration claims that the new rule does not significantly reduce protections, in fact millions of acres of wetlands nationwide will be affected,” Bob Irvin, president and CEO of American Rivers, said in a statement. “This is an early Christmas gift to polluters and a lump of coal for everyone else,” he added.

The announcement is a significant step in the Trump administration’s efforts to minimize the authority of the EPA, as well as a victory for anti-regulation Republicans and the agricultural industry. The change in regulatory language, which carries with it alarming implications, comes after Trump’s repeated rhetoric that the United States needs “crystal-clean water.”

The redefinition of Waters of the United States (WOTUS) will dampen the efficacy of the Clean Water Act, a federal law implemented by President Richard Nixon in 1972 that aims to control pollution in U.S. waterways. In 2015, President Obama tried to protect smaller waterways like streams and wetlands under the Clean Water Rule, and was met with fierce resistance from Republicans and farmers who complained that the rule was too far-reaching.

The EPA’s announcement of the new definition included language pointed directly at the former president. “Unlike the Obama administration’s 2015 definition... today’s proposal contains a straightforward definition that would result in significant cost savings, protect the nation’s navigable waters, help sustain economic growth, and reduce barriers to business development,” reads a statement released by the federal agency.

EPA Office of Water chief Dave Ross told reporters Monday evening that the the agency did not have access to—and therefore did not consult—data that shows how many U.S. waterways would be affected by the new definition.

“We have not done... a detailed mapping of all the wetlands in the country,” Ross said. “No one has that data.”

However, according to internal EPA slides obtained by E&E News through a federal records request, the agency did have access to that data as long as a year ago. The uncovered slides were prepared for former EPA Administrator Scott Pruitt and former Army Corps Deputy Assistant Secretary Douglas Lamont by EPA and Army Corps of Engineers staff in 2017.

The slides include data from the U.S. Geological Survey’s National Hydrography Dataset (NHD)—which maps streams and gauges how often they flow—and show that 51 percent of wetlands may become unregulated under the new WOTUS definition.

It appears the slides were presented when the Trump administration was deciding whether to include ephemeral streams—streams that only exist after rain or snowmelt—in a new WOTUS definition. Late Supreme Court Justice Antonin Scalia wrote in a 2006 court ruling that only waterways with a “relatively permanent flow” should be covered by the Clean Water Act, an opinion the Trump administration appears to be following up on now, as ephemeral streams will be no longer protected under the scaled-back regulation.

Environmental advocates argue that eliminating ephemeral streams from federal protection could create more downstream pollution, as streams that have been refilled in the rainy season may transfer agricultural pollution to larger bodies of water.

“Saying you want clean water and excluding ephemeral streams is like saying you want universal health care, but you won’t cover anyone not named Ken,” said Ken Kopocis, who led the Obama EPA’s Office of Water. “It’s a start, but it won’t get you what you want.”

The reclassification of ephemeral streams could be especially important in western states, like California, where most streams are seasonal by nature and agriculture is the top industry.